Cover rock breakthrough pressure testing device and method based on micro-strain synchronous sensing

Abstract

The invention discloses a cover rock breakthrough pressure testing device and method based on micro-strain synchronous sensing, and the device integrates a core clamping module, a stress loading module, a temperature control module, a gas injection module, a strain sensing module and a flow monitoring module. Interference of gas displacement water on gas breakthrough identification is eliminated by adopting a multiphase flow isolation design, an optical fiber layout architecture is innovatively proposed to solve the problem of signal interference in traditional rock core gas breakthrough monitoring, and a dynamic uniform temperature field with a gradient compensation function is constructed in combination with a distributed temperature control array embedded in a holder. The breakthrough of gas from the rock core is captured in real time by using the micro-strain signal evolved by gas pressure induction, the breakthrough of the gas can be monitored in the first time, and the testing precision of the breakthrough pressure is improved. The method breaks through the limitation of traditional rock core specifications, can make full use of irregular broken rock samples, successfully solves the test bottleneck caused by insufficient on-site coring integrity, and expands the engineering application boundary of a cap layer sealing performance evaluation technology.

Description

Technical Field

[0001] The present invention belongs to the technical field of CO2 breakthrough pressure testing of cap rock, and in particular relates to a device and method for testing the CO2 breakthrough pressure of cap rock based on microstrain synchronous sensing. Background Art

[0002] Carbon capture and storage (CCS) is a globally recognized, rapidly implemented, and highly effective method for reducing CO2 emissions. It involves injecting captured CO2 into specific geological structures for storage, effectively isolating it from the atmosphere. The promise of large-scale CO2 geological storage relies on its long-term safety and stability. While ensuring sufficient storage capacity, CO2 geological storage sites also require caprocks with excellent integrity and sealing properties. Low-permeability caprocks (such as salt rock, gypsum, shale, and tight sandstone) directly overlying the reservoir are the primary barrier to leakage of injected CO2. To maximize storage capacity, continuous CO2 injection inevitably leads to a buildup of formation pressure. While this increasing formation pressure ensures a significant swept volume and enhanced storage quality, it also increases the risk of leakage from the overburden. This dynamic trade-off between increasing reservoir pressure and the sealing properties of the caprock is a major constraint on storage safety. Accurately evaluating the sealing performance of caprock has become an essential step in the implementation of CCS projects, and is also the key to conducting suitability evaluation and optimization screening of storage sites.

[0003] Breakthrough pressure is the pressure threshold that characterizes the sealing ability of rock and is a key indicator for evaluating the sealing performance of caprock. Core-scale breakthrough pressure test can simulate in-situ formation conditions and is the most commonly used method to directly test rock breakthrough pressure. However, in the experiment of testing rock gas breakthrough pressure, the following problems still exist: (1) The judgment standard of gas breakthrough. At present, the continuous escape of bubbles or downstream pressure changes observed in the flow detection device at the outlet of the clamp are usually used as signs to judge that gas has broken through the core. However, the gas breaking through the core flows slowly and can only be detected when it flows continuously to the flow detection device. Especially when the outlet is set with back pressure, the gas will not flow out from the downstream back pressure valve and be detected until the gas pressure accumulates at the outlet and reaches the back pressure value. This method cannot detect gas breakthrough in the first time, which may cause the breakthrough pressure value to be overestimated. How to detect gas breakthrough from the core at the first moment is the key to accurately testing breakthrough pressure. The synchronous perception of microstrain caused by gas breakthrough at the outlet of the core is an effective way to accurately identify the moment of gas breakthrough. Chinese patent CN 115248177A discloses a method and device for measuring the breakthrough pressure of low-permeability rocks based on fiber optic sensing. Fiber optic sensing is applied to the test of rock breakthrough pressure, and the sensitivity of optical fiber to temperature and strain detection is used to monitor the breakthrough of gas from the core. However, this method still has the following shortcomings and cannot effectively guide actual experiments: ① The layout of the optical fiber. The strain signal of the optical fiber arranged along the circumference and axial direction of the core forms a coupling effect with the end-face monitoring channel, making it difficult to trace the strain response. Specifically, the system cannot distinguish whether the monitoring signal is caused by the strain caused by gas migration inside the core, or the sudden change in end-face stress induced by gas breakthrough at the outlet. At the same time, the baseline strain amplitude caused by the core circumferential optical fiber under the action of confining pressure load is significantly higher than the strain signal generated by the instantaneous pressure fluctuation of trace gas breakthrough. This magnitude difference makes the characteristic strain at the breakthrough moment easily obscured by the system noise and the background signal of the confining pressure, resulting in failure to capture the dynamic response; ② The influence of gas-driven water on strain monitoring is not considered. The gas will displace the water in the core before the breakthrough, and it is impossible to distinguish whether the monitored strain is caused by the flow of displaced water or the breakthrough of gas at the outlet, and the breakthrough moment of the gas cannot be accurately identified; ③ Its heating method (water bath heating) has a significant thermal boundary layer effect, low heat conduction efficiency, large temperature gradient inside and outside the clamp, and temperature changes interfere with the identification of the gas breakthrough moment. (2) The existing breakthrough pressure test method has high requirements for the core specifications. The current method requires the use of cylindrical cores with no obvious cracks in the cores. However, it is often difficult to obtain complete cores in actual engineering, and irregularly broken cores are common. In this case, how to make full use of irregularly broken cores to obtain rock breakthrough pressure is also a difficult problem faced in the current evaluation of caprock sealing performance.

[0004] Therefore, in response to the above problems, how to develop new testing methods and devices to accurately characterize the rock breakthrough pressure has become the focus of research and technology development on the evaluation of caprock sealing performance. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to address the deficiencies of the above-mentioned existing technologies and provide a device and method for testing the breakthrough pressure of cap rock based on synchronous microstrain sensing, which can simulate the in-situ formation temperature and pressure conditions. The synchronous sensing of the microstrain caused by the gas at the core outlet can identify the breakthrough of gas from the core in the first time, thereby improving the test accuracy of the breakthrough pressure. At the same time, this method can make full use of irregularly broken cores, effectively solving the problem of test failure caused by the inability to obtain regular and complete core samples in actual engineering scenarios.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is:

[0007] A caprock breakthrough pressure testing device based on micro-strain synchronous sensing includes a core clamping module, a stress loading module, a temperature control module and a gas injection module; the core clamping module includes a temperature and pressure resistant sleeve, a heat shrink tube resistant to CO2 corrosion is installed in the temperature and pressure resistant sleeve, and a porous pad, a core and a special core are arranged in sequence from bottom to top in the heat shrink tube; the porous pad is connected to the gas injection pipeline through a gas injection pipeline, an inlet pressure sensor is provided on the gas injection pipeline, and the gas injection module is used to inject CO2 into the porous pad; the special core has a built-in water-absorbing material to absorb water displaced by the gas, a fiber optic Bragg grating is provided on the upper end face of the special core, the fiber optic Bragg grating is connected to the fiber optic demodulator through an optical fiber lead, and the special core is connected to the gas-liquid outflow pipeline. The gas-liquid outflow pipeline is connected, and an outlet pressure sensor and a gas flow meter are provided on the gas-liquid outflow pipeline; the stress loading module includes an injection pump, an annular pressure gauge, a back pressure valve and a back pressure gauge, and the injection pump is connected to the gap between the temperature-resistant pressure sleeve and the heat shrink tube through the annular pressure pipeline, the annular pressure gauge is arranged on the annular pressure pipeline, the back pressure valve is arranged on the gas-liquid outflow pipeline, and the back pressure valve is connected to the injection pump through the back pressure pipeline, and the back pressure gauge is arranged on the back pressure pipeline; the temperature control module includes a micro heating wire and a temperature controller, the micro heating wire is woven into a mesh and wrapped around the side wall of the porous pad, core or special core to form a group of temperature control units, and several groups of independent temperature control units are arranged along the axial direction to form a distributed heat source array, and the external lead is connected to the temperature controller.

[0008] In the above solution, a branch is further provided on the gas injection pipeline to connect with a vacuum pump.

[0009] In the above scheme, the special core is cylindrical, the upper and lower parts of the special core are solid structures, and the middle part is a cavity structure, and the cavity structure is filled with water-absorbing material; a number of fluid microchannels connected to the cavity structure are evenly opened in the lower part of the special core, so that the fluid discharged from the core can flow evenly into the special core; a series of gas microchannels are opened in the longitudinal and transverse directions on the upper part of the special core, and the fiber grating is arranged on the upper end face of the gas microchannel. After the dried CO2 gas passes through the gas microchannel, it flows evenly through the fiber grating.

[0010] In the above solution, the water absorbing material is silica gel particles.

[0011] In the above solution, a plurality of through holes are evenly arranged in the porous spacer along the axial direction of the core for gas to pass through.

[0012] In the above solution, the core is prepared from a regular complete core, and the regular complete core is processed into a cylindrical core.

[0013] In the above scheme, the core is prepared from irregular fractured cores, and the following preparation method is adopted: irregular core fragments without obvious cracks are selected and placed in a constant temperature drying oven for drying, and the temperature is set to 85-105°C to ensure that the pore structure of the core fragments is not destroyed; the irregular core fragments are placed in a mold so that the core fragments are completely immersed in high-strength geopolymer material. After injection molding, the composite core is processed into a cylindrical shape; and holes are drilled into the rock sample along the center of the top and bottom of the cylindrical composite core to form a gas flow channel.

[0014] In the above solution, the gas injection module includes a gas tank and an ISCO plunger pump. The outlet of the gas tank is connected to the inlet of the ISCO plunger pump through a gas injection pipeline. A gas tank pressure gauge is provided on the gas injection pipeline. The outlet of the ISCO plunger pump is connected to the gas injection pipeline.

[0015] In the above solution, the micro heating wire is a carbon fiber heating wire or a nickel-chromium alloy heating wire.

[0016] Accordingly, the present invention also proposes a method for testing the breakthrough pressure of cap rock based on synchronous microstrain sensing. The above-mentioned testing device includes the following steps:

[0017] S1. Core preparation: Use regular intact cores or irregular fractured cores to prepare cores;

[0018] S2. Core saturation treatment: According to the formation water information of the stratum where the core is located, the formation water saturated core is configured;

[0019] S3. In-situ formation environment simulation: The saturated core is wrapped with heat shrink tubing and placed in a temperature- and pressure-resistant sleeve. A porous spacer is placed at the core inlet, allowing gas to enter the core through the through-holes in the porous spacer. A special core is placed at the core outlet. The special core has built-in water-absorbing material to absorb water displaced by the gas, and a fiber Bragg grating is installed on the upper end face of the special core. Then, an annular pressure is applied around the core, and the core surface is heated until the core temperature exceeds 31.1°C to simulate the in-situ formation stress and temperature conditions. A certain back pressure is applied to the outlet of the core clamping module, with a back pressure value greater than 7.31MPa to ensure that the CO2 is in a supercritical state.

[0020] S4. Vacuum treatment: Use a vacuum pump to extract the air in the core, pipelines and core clamping module to eliminate the influence of air on the experimental results;

[0021] S5. Monitoring of gas breakthrough process: CO2 is injected into the core inlet at an initial pressure, and the gas pressure is gradually increased, providing a sufficiently long time interval at each constant pressure. The pressure interval and constant pressure time are determined according to the method for measuring the breakthrough pressure of rock gas. During the experiment, the annular pressure is dynamically adjusted according to the injected gas pressure to keep the net annular pressure equal to the overlying stress of the formation. The breakthrough moment of the gas in the core is monitored by the strain change reflected by the fiber Bragg grating. The measurement difference between the inlet pressure sensor and the outlet pressure sensor corresponding to the breakthrough moment is the breakthrough pressure.

[0022] The beneficial effects of the present invention are:

[0023] 1. This invention solves the signal interference problem in traditional core gas breakthrough monitoring through an innovative fiber optic layout architecture. Specifically, unlike conventional circumferential / axial fiber optic layout schemes, this invention integrates a high-sensitivity fiber Bragg grating sensor at the top of a special core. This design has two technical advantages: (1) It avoids the baseline strain noise caused by conventional circumferential layout, so that the signal-to-noise ratio of the strain signal caused by the micro-pressure fluctuations generated by the trace gas breakthrough is improved; (2) It accurately traces the strain response: the high-permeability special core has a fast flow-guiding characteristic. After breaking through the core, the gas can quickly pass through it and reach the optical fiber, causing the optical fiber wavelength to change. Based on the geometric correspondence between the breakthrough front migration path and the optical fiber topology, the dynamic capture of the gas breakthrough front is achieved.

[0024] 2. The present invention adopts a sophisticated design to eliminate the interference of water displaced by gas on gas breakthrough identification. The breakthrough of gas from the core is characterized by the synchronous perception of micro-strain caused by gas pressure. The measurement sensitivity is high, and gas breakthrough can be monitored in the first time, thereby improving the test accuracy of breakthrough pressure.

[0025] 3. This invention innovatively integrates a distributed array temperature control module within the core holder, creating a dynamic, uniform temperature field with gradient compensation, overcoming the limitations of traditional external heating solutions. Compared to traditional external heating solutions, this technology significantly improves thermodynamic efficiency, shortens thermal response time, and maintains temperature fluctuations of ≤±0.05°C. This technology eliminates fiber strain noise caused by substrate temperature fluctuations, improving the accuracy of gas breakthrough detection.

[0026] 4. The present invention breaks through the geometric constraint bottleneck of traditional core testing. The proposed method has low requirements for core specifications and can make full use of irregularly broken cores. It can effectively solve the problem of test failure caused by the inability to obtain regular and complete core samples in actual engineering scenarios, expand the scope of experimental testing, and enhance the breadth of application. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0028] Figure 1 This is a schematic diagram of the overall structure of the cap rock breakthrough pressure testing device based on micro-strain synchronous sensing of the present invention;

[0029] Figure 2 It is a schematic diagram of the special core design of the present invention.

[0030] In the figure: 1. Gas tank; 2. Gas tank pressure gauge; 3. ISCO plunger pump; 4. Vacuum pump; 5. Three-way valve; 6. Inlet pressure sensor; 7. Lower bracket; 8. Porous spacer; 9. Temperature and pressure resistant sleeve; 10. Heat shrink tubing; 11. Core; 12. Special core; 121. Silica gel particles; 122. Fluid microchannel; 123. Gas microchannel; 13. Fiber Bragg grating (FBG); 14. Upper bracket; 15. Micro heating wire; 16. Temperature controller; 17. Fiber demodulator; 18. Gas flow meter; 19. Outlet pressure sensor; 20. Back pressure valve; 21. Back pressure pressure gauge; 22. Ring pressure pressure gauge; 23. Three-way valve; 24. Injection pump. DETAILED DESCRIPTION

[0031] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, specific embodiments of the present invention are now described in detail with reference to the accompanying drawings.

[0032] like Figure 1 As shown, a cap rock breakthrough pressure testing device based on micro-strain synchronous sensing provided by an embodiment of the present invention includes a core clamping module, a stress loading module, a temperature control module, a vacuum module, a gas injection module, a strain monitoring module and a flow monitoring module.

[0033] The core clamping module is the main place for simulating gas breakthrough in the core. It includes a temperature and pressure resistant sleeve 9, in which a heat shrink tube 10 resistant to CO2 corrosion is installed. The heat shrink tube 10 is tightly wrapped with a porous pad 8, a core 11, and a special core 12 from bottom to top. The porous pad 8 is connected to the gas injection module through a gas injection pipeline. The gas injection pipeline is provided with an inlet pressure sensor 6. The gas injection module is used to inject CO2 into the porous pad 8. The special core 12 has built-in water-absorbing material to absorb water displaced by the gas, eliminating the influence of water on gas breakthrough identification. A fiber grating 13 is provided on the upper end face of the special core 12. The fiber grating 13 is connected to the fiber demodulator 17 through an optical fiber lead. The special core 12 is connected to the gas-liquid outflow pipeline. The gas-liquid outflow pipeline is provided with an outlet pressure sensor 19 and a gas flow meter 18.

[0034] The stress loading module is used to simulate the in-situ formation stress conditions of the core 11 and includes an injection pump 24, an annular pressure gauge 22, a back-pressure valve 20, and a back-pressure gauge 21. The injection pump 24 is connected to the gap between the temperature-resistant and pressure-resistant sleeve 9 and the heat-shrink tubing 10 via an annular pressure line and a three-way valve 23. The annular pressure gauge 22 is installed on the annular pressure line. The back-pressure valve 20 is installed on the gas-liquid outflow line and is connected to the injection pump 24 via the back-pressure line and the three-way valve 23. The back-pressure gauge 21 is also installed on the back-pressure line. The injection pump 24 and the annular pressure gauge 22 constitute the annular pressure loading unit. The injection pump 24 injects water into the gap between the temperature-resistant and pressure-resistant sleeve 9 and the heat-shrink tubing 10 to provide annular pressure. The injection pump 24, the back-pressure valve 20, and the back-pressure gauge 21 constitute the back-pressure loading unit. The injection pump 24 injects water into the back-pressure valve 20 to provide back pressure. In one embodiment, the injection pump 24 is a hand-cranked injection pump.

[0035] The temperature control module utilizes internal heating within the holder and comprises micro-heating wires 15 and a temperature controller 16. The micro-heating wires 15 are woven into a mesh and adhered to the sidewalls of the porous block 8, core 11, or custom core 12, forming a group of temperature control units. Several independent temperature control units are arranged axially to form a distributed heat source array. External leads connect to the temperature controller 16, which controls the in-situ formation temperature within the holder. This arrangement of the temperature control module overcomes the limitations of external heating solutions. By creating a dynamic uniform temperature field with gradient compensation, thermodynamic efficiency is significantly improved, with temperature fluctuations ≤±0.05°C. It also eliminates fiber strain noise caused by substrate temperature fluctuations, improving the accuracy of gas breakthrough detection. In one embodiment, the micro-heating wires 15 are carbon fiber or nickel-chromium alloy heating wires, with five to six independent temperature control units arranged axially along the porous block 8, core 11, and custom core 12.

[0036] The vacuum module includes a vacuum pump 4, which is connected to the gas injection pipeline via a three-way valve 5. The vacuum pump 4 is used to remove air from the core 11, the core clamping module and the pipelines to eliminate the interference of air on the breakthrough of CO2 gas.

[0037] The gas injection module is used to inject CO2 gas into the core 11 at a constant pressure or constant rate, and includes a gas tank 1 and an ISCO plunger pump 3. The outlet of the gas tank 1 is connected to the inlet of the ISCO plunger pump 3 through a gas injection pipeline. The gas injection pipeline is provided with a gas tank pressure gauge 2. The outlet of the ISCO plunger pump 3 is connected to the gas injection pipeline through a three-way valve 5.

[0038] The strain monitoring module, used to monitor gas breakthrough in the core 11, includes a fiber Bragg grating (FBG) 13 and a fiber interrogator 17. The FBG 13 is attached to the upper end face of the custom core 12. An optical cable is routed through the outlet and connected to the fiber interrogator 17, which records the signal. The strain changes synchronously monitored by the FBG 13 are used to determine the moment of gas breakthrough in the core 11.

[0039] The flow monitoring module is used to test the flow rate after gas breakthrough, and includes an outlet pressure sensor 19 and a gas flow meter 18. The gas flow meter 18 is connected to the upper end of the back pressure valve 20 through the gas-liquid outflow pipeline to test the flow rate of gas after breaking through the core 11.

[0040] like Figure 2 As shown, the custom core 12 is cylindrical, specifically made of a temperature- and pressure-resistant material such as stainless steel. The upper and lower portions of the custom core 12 are solid structures, while the middle portion is a hollow structure. The hollow structure is filled with silica gel particles 121 to absorb water displaced by the gas, thereby eliminating the effect of water on gas breakthrough detection. Several fluid microchannels 122 are evenly distributed in the lower portion of the custom core 12, connecting to the hollow structure. This allows the fluid displaced from the core to flow evenly into the custom core 12, thereby allowing the displaced water to be adsorbed by the silica gel particles 121. A series of gas microchannels 123 are provided in the upper portion of the custom core 12 in both the longitudinal and transverse directions. Fiber Bragg gratings 13 are arranged on the upper end surfaces of the gas microchannels 123. The dried CO2 gas passes through the gas microchannels 123 and then evenly flows through the fiber Bragg gratings 13.

[0041] In one embodiment, the special core 12 is made of stainless steel, has a diameter of 2.5 cm and a length of 2 cm. A series of fluid microchannels 122 with a diameter of 3 mm are evenly opened at the lower part of the special core 12, and a series of gas microchannels 123 with a diameter of 3 mm are evenly opened in the longitudinal and transverse directions at the upper part of the special core 12.

[0042] Further optimized, the porous spacer 8 has high porosity and good permeability, ensuring that the injected gas can evenly contact the surface of the core 11. Specifically, a plurality of through holes are provided in the porous spacer 8 along the axial direction of the core 11 for gas to pass through.

[0043] Further optimization, the core 11 is prepared from a regular complete core 11 or an irregular fractured core 11:

[0044] For the regular intact core 11, the cap rock is processed into a cylindrical core 11;

[0045] The following preparation method is adopted for the irregular fractured rock core 11: ① Irregular rock core 11 fragments without obvious cracks are selected and placed in a constant temperature drying oven for drying at a temperature set at 85-105°C to ensure that the pore structure of the rock core 11 fragments is not destroyed; ② The irregular rock core 11 fragments are placed in a mold so that the rock core 11 fragments are completely immersed in a high-strength geopolymer material. After injection molding, the composite rock core 11 is processed into a cylindrical shape; ③ Holes are drilled along the center of the top and bottom of the cylindrical composite core 11 into the rock sample to form a gas flow channel.

[0046] Further optimization, the core clamping module also includes a lower bracket 7 and an upper bracket 14, the temperature and pressure resistant sleeve 9 is arranged between the lower bracket 7 and the upper bracket 14, the lower bracket 7 is provided with a hole for the gas injection pipeline to pass through, and the upper bracket 14 is provided with holes for the gas and liquid outflow pipelines and optical fiber leads to pass through respectively.

[0047] Accordingly, the present invention also proposes a method for testing the breakthrough pressure of cap rock based on synchronous microstrain sensing, which uses the above-mentioned testing device and includes the following steps:

[0048] S1. Preparation of core 11:

[0049] For the regular complete core 11 , the cap rock is processed into a cylindrical core 11 with a diameter of 2.5 cm and a length of 5 cm.

[0050] For the irregular crushed core 11, the following preparation method is adopted:

[0051] ① Irregular core 11 fragments without obvious cracks were selected and placed in a constant temperature drying oven for drying at 85°C to ensure that the pore structure of the core 11 fragments was not destroyed;

[0052] ② Aluminum dihydrogen phosphate and fly ash are mixed and reacted at a certain liquid-to-solid ratio (preferably 0.32, whose compressive strength can reach 53 MPa) to obtain a high-strength geopolymer material. The irregular core 11 fragments are placed in a mold. The core 11 is completely immersed in the high-strength geopolymer material. After injection molding, the composite core 11 is processed into a cylindrical shape with a diameter of 2.5 cm and a length of 5 cm.

[0053] ③ Drill holes along the center of the top and bottom of the cylindrical core 11 into the rock sample to form a gas flow channel.

[0054] S2. Water saturation treatment of core 11: Based on the formation water information of the stratum where core 11 is located, core 11 is saturated with formation water. The water saturation process is carried out in accordance with the "Analysis Method of Core 11" GB / T 29172.

[0055] S3. In-situ formation environment simulation: Wrap the saturated core 11 with a heat shrink tube 10 and place it in a temperature and pressure resistant sleeve 9. Place a porous pad 8 at the inlet end of the core 11 so that gas can enter the core 11 through the through hole in the middle of the porous pad 8. Arrange a special core 12 at the outlet end of the core 11. The special core 12 has built-in water-absorbing material to absorb water displaced by the gas. A fiber Bragg grating 13 is provided on the upper end face of the special core 12. Then, apply an annular pressure around the core 11 and apply a certain temperature (greater than 31.1°C) to the core 11 to simulate the in-situ formation stress and temperature conditions. Apply a certain back pressure at the outlet end of the core clamping module. The back pressure value is greater than 7.31MPa to ensure that the CO2 is in a supercritical state.

[0056] S4, vacuum treatment: The air in the core 11, each pipeline and the core clamping module is extracted by the vacuum pump 4 to eliminate the influence of the air on the experimental results.

[0057] S5. Monitoring of gas breakthrough process: CO2 is injected into the inlet of the core 11 at a relatively low initial pressure, and the gas pressure is gradually increased, providing a sufficiently long time interval at each constant pressure. The pressure interval and constant pressure time are determined according to the method for determining the breakthrough pressure of rock gas (Petroleum and Natural Gas Industry Standard, SY / T5748-2020); in the experiment, the annular pressure is dynamically adjusted according to the injected gas pressure to keep the net annular pressure equal to the overlying stress of the formation; the breakthrough of the gas in the core 11 is monitored by the strain change reflected by the fiber grating 13. Specifically, the moment when the fiber optic demodulator 17 detects the strain change is the CO2 breakthrough moment, and the measurement difference between the inlet pressure sensor 6 and the outlet pressure sensor 19 corresponding to this moment is the breakthrough pressure.

[0058] Theoretically, when the gas pressure reaches the capillary entry pressure, the gas invades the core 11 and displaces the water in the core 11. Until the gas pressure overcomes the breakthrough pressure, the gas breaks through the core 11 and the gas continuously escapes. The passage of gas through the optical fiber will cause a change in the length of the fiber grating 13. The change in the reflected wavelength is measured by the optical fiber demodulator 17, which in turn reflects the strain change. In order to avoid interference with strain monitoring due to water flow, the water displaced from the core 11 can be adsorbed by the silica gel particles 121 filled in the cavity of the special core 12. Based on the interpretation of the optical fiber signal on the upper side of the special core 12, when a strain change is detected, the gas breaks through the core 11. At this time, the corresponding pressure difference at both ends of the core clamping module is the CO2 breakthrough pressure of the core 11. The gas flow after the gas breakthrough is measured by the gas flowmeter 18.

[0059] The axial strain of the optical fiber under pressure (ε z ) and reflection wavelength (λ Β ) can be calculated by the following formula:

[0060]

[0061] Δλ B =(1-P e )λ B ε z (2)

[0062]

[0063] Where: P is the gas pressure, Δλ B is the change in reflection wavelength, v is the Poisson's ratio of the optical fiber, E is the elastic modulus of the optical fiber, P 11 and P 12 is the elastic-optical coefficient of the optical fiber material, P e is the effective elastic-optical coefficient, η eff is the effective refractive index of the optical fiber.

[0064] From the above, the present invention uses optical fiber to monitor the microstrain changes caused by gas pressure to characterize the breakthrough of gas from the core 11. It has the advantages of high measurement accuracy and support for real-time monitoring and analysis. It can monitor the breakthrough of gas in the first time and improve the test accuracy of the breakthrough pressure. The silica gel particles 121 in the internal cavity of the special core 12 can adsorb the water displaced from the core 11 and eliminate the interference of water flow on the microstrain changes caused by gas breakthrough. By wrapping the broken core 11 with high-strength geopolymer material, it is possible to ensure that the gas breaks through the core 11 without destroying the pore structure of the core 11, and then obtain the breakthrough pressure of the core 11. This method can make full use of irregular fractured cores 11, and can effectively solve the problem of not being able to obtain regular and complete core 11 samples in actual engineering scenarios, thereby expanding the scope of experimental testing.

[0065] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0066] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims, all of which are protected by the present invention.

Claims

1. A caprock breakthrough pressure testing device based on micro-strain synchronous sensing, comprising a core clamping module, a stress loading module, a temperature control module and a gas injection module; characterized in that: The core clamping module includes a temperature and pressure resistant sleeve, a heat shrink tube resistant to CO2 corrosion is installed in the temperature and pressure resistant sleeve, and a porous pad, a core, and a special core are arranged in sequence from bottom to top in the heat shrink tube; the porous pad is connected to the gas injection module through a gas injection pipeline, and an inlet pressure sensor is provided on the gas injection pipeline. The gas injection module is used to inject CO2 into the porous pad; the special core has a built-in water-absorbing material to absorb water expelled by the gas, and a fiber grating is provided on the upper end face of the special core, and the fiber grating is connected to the core through an optical fiber. The lead is connected to the optical fiber demodulator, and the special core is connected to the gas-liquid outflow pipeline, on which an outlet pressure sensor and a gas flow meter are provided; the stress loading module includes an injection pump, an annular pressure gauge, a back pressure valve and a back pressure gauge, the injection pump is connected to the gap between the temperature-resistant pressure sleeve and the heat shrink tube through the annular pressure pipeline, the annular pressure gauge is arranged on the annular pressure pipeline, the back pressure valve is arranged on the gas-liquid outflow pipeline, and the back pressure valve is connected to the injection pump through the back pressure pipeline, and the back pressure gauge is arranged on the back pressure pipeline; The temperature control module includes a micro heating wire and a temperature controller. The micro heating wire is woven into a mesh and pasted around the side wall of a porous pad, a rock core or a special rock core to form a group of temperature control units. Several groups of independent temperature control units are arranged along the axial direction to form a distributed heat source array, and the external leads are connected to the temperature controller.

2. The device for testing the breakthrough pressure of cap rock based on synchronous microstrain sensing according to claim 1 is characterized in that: The gas injection pipeline is also provided with a branch line connected to a vacuum pump.

3. The device for testing the breakthrough pressure of cap rock based on synchronous microstrain sensing according to claim 1 is characterized in that: The special core is cylindrical, with the upper and lower parts of the special core being solid structures and the middle part being a cavity structure, and the cavity structure being filled with water-absorbing material; a plurality of fluid microchannels connected to the cavity structure are evenly opened in the lower part of the special core to facilitate the uniform flow of fluid discharged from the core into the special core; a series of gas microchannels are opened in the longitudinal and transverse directions on the upper part of the special core, and the fiber grating is arranged on the upper end face of the gas microchannel, and the dried CO2 gas flows evenly through the fiber grating after passing through the gas microchannel.

4. The device for testing the breakthrough pressure of cap rock based on synchronous microstrain sensing according to claim 3 is characterized in that: The water absorbing material is silica gel particles.

5. The device for testing the breakthrough pressure of cap rock based on synchronous microstrain sensing according to claim 1 is characterized in that: A plurality of through holes are evenly arranged in the porous pad along the axial direction of the core for gas to pass through.

6. The device for testing the breakthrough pressure of cap rock based on synchronous microstrain sensing according to claim 1 is characterized in that: The rock core is prepared from a regular complete rock core, which is processed into a cylindrical rock core.

7. The device for testing the breakthrough pressure of cap rock based on synchronous microstrain sensing according to claim 1 is characterized in that: The core is prepared from irregular fractured cores using the following preparation method: irregular core fragments without obvious cracks are selected and placed in a constant temperature drying oven for drying at a temperature set at 85 to 105° C. to ensure that the pore structure of the core fragments is not destroyed; the irregular core fragments are placed in a mold so that the core fragments are completely immersed in a high-strength geopolymer material; after injection molding, the composite core is processed into a cylindrical shape; and holes are drilled into the center of the top and bottom of the cylindrical composite core to enter the rock sample to form a gas flow channel.

8. The device for testing the breakthrough pressure of cap rock based on synchronous microstrain sensing according to claim 1 is characterized in that: The gas injection module includes a gas tank and an ISCO plunger pump. The outlet of the gas tank is connected to the inlet of the ISCO plunger pump through a gas injection pipeline. A gas tank pressure gauge is provided on the gas injection pipeline. The outlet of the ISCO plunger pump is connected to the gas injection pipeline.

9. The device for testing the breakthrough pressure of cap rock based on synchronous microstrain sensing according to claim 1, characterized in that: The micro heating wire is a carbon fiber heating wire or a nickel-chromium alloy heating wire.

10. A method for testing the breakthrough pressure of caprock based on synchronous microstrain sensing, characterized in that: The testing device according to any one of claims 1 to 9 comprises the following steps: S1. Core preparation: Use regular intact cores or irregular fractured cores to prepare cores; S2. Core saturation treatment: According to the formation water information of the stratum where the core is located, the formation water saturated core is configured; S3. In-situ formation environment simulation: The saturated core is wrapped with heat shrink tubing and placed in a temperature- and pressure-resistant sleeve. A porous spacer is placed at the core inlet, allowing gas to enter the core through the through-holes in the porous spacer. A special core is placed at the core outlet. The special core has built-in water-absorbing material to absorb water displaced by the gas, and a fiber Bragg grating is installed on the upper end face of the special core. Then, an annular pressure is applied around the core, and the core surface is heated until the core temperature exceeds 31.1°C to simulate the in-situ formation stress and temperature conditions. A certain back pressure is applied to the outlet of the core clamping module, with a back pressure value greater than 7.31MPa to ensure that the CO2 is in a supercritical state. S4. Vacuum treatment: Use a vacuum pump to extract the air in the core, pipelines and core clamping module to eliminate the influence of air on the experimental results; S5. Monitoring of gas breakthrough process: CO2 is injected into the core inlet at an initial pressure, and the gas pressure is gradually increased, providing a sufficiently long time interval at each constant pressure. The pressure interval and constant pressure time are determined according to the method for measuring the breakthrough pressure of rock gas. During the experiment, the annular pressure is dynamically adjusted according to the injected gas pressure to keep the net annular pressure equal to the overlying stress of the formation. The breakthrough moment of the gas in the core is monitored by the strain change reflected by the fiber Bragg grating. The measurement difference between the inlet pressure sensor and the outlet pressure sensor corresponding to the breakthrough moment is the breakthrough pressure.

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