Method and device for measuring breakthrough pressure of low-permeability cap layer based on absorption spectrum sensing

Abstract

The invention discloses a method and a device for measuring the breakthrough pressure of a low-permeability cap based on absorption spectrum sensing. The measuring device comprises a confining pressure injection module, a data monitoring module, a return pressure setting module, an absorption spectrum sensing module, a rock core holder module and a pore pressure injection module. According to the invention, the breakthrough pressure of the low-permeability cap layer can be tested, the test work can be reliably and efficiently completed, and a pressure signal and an absorption spectrum signal are comprehensively utilized; the absorption spectrum is innovatively applied to the breakthrough pressure test, the rock sample breakthrough moment can be accurately and effectively judged, the sensitivity limitation and delay of pressure monitoring in the low-permeability cover layer test are avoided, and a good detection and tracking system is provided for breakthrough pressure calculation, breakthrough position and form change of displacement fluid; meanwhile, according to the gas monitoring function, the process and efficiency of gas breaking through the rock core can be monitored in real time to a certain extent.

Description

Technical Field

[0001] The present invention relates to the technical field of carbon dioxide geological storage and cap rock sealing evaluation of underground gas storage, and in particular to a method for measuring the breakthrough pressure of a low permeability cap rock based on absorption spectrum sensing, and also to a device for measuring the breakthrough pressure of a low permeability cap rock based on absorption spectrum sensing, which is suitable for rapid, accurate and efficient measurement of the breakthrough pressure of cap rocks of carbon dioxide geological storage and gas storage. Background Art

[0002] Carbon capture, utilization and storage (CCUS) technology is a key technical means and bottom-line technical guarantee for temperature control targets. Among them, for related fields such as oil and gas reservoirs, geological storage of carbon dioxide, underground gas storage, and deep geological storage of nuclear waste, the evaluation of storage potential and cap rock tightness is the current research hotspot and key. Therefore, how to accurately and efficiently evaluate the sealing capacity of the cap rock has important research value.

[0003] The breakthrough pressure is defined as the resistance to fluid migration that the non-wetting phase overcomes when it first breaks through the interconnected pores of the caprock, and its magnitude is equal to the sum of the capillary pressure, adsorption resistance, and viscous resistance of the largest connected pore in the rock. When the gas pressure is greater than the breakthrough pressure of the caprock, a completely connected seepage channel is formed inside the caprock, so that gas seepage and leakage occur. Therefore, the breakthrough pressure (displacement pressure) is the most objective and intuitive numerical indicator to measure the tightness of the caprock. Although there are various breakthrough pressure testing devices, there are still various limitations, such as how to test fault rocks, how to refine the breakthrough time, and how to monitor the migration and breakthrough process of gas.

[0004] Earlier studies on the evaluation methods of the sealing capacity of oil and gas reservoir caprocks pointed out that the caprock displacement pressure obtained by the direct displacement method can best reflect the actual physical sealing capacity of the caprock (Jiang Zhenxue et al., Study on the Gas Sealing Effectiveness of the Mudstone Caprock of the Qingshankou Formation in Sanzhao Area, Journal of Daqing Petroleum Institute, 1996). However, the breakthrough pressure measured by the existing experimental device is often far from the theoretical value and the actual value, mainly due to the insufficient accuracy of the overall device, insufficient monitoring of small gas pressure changes, and delayed gas migration caused by the long pipeline. Therefore, in actual testing, whether it is through the gas pressure curve or the tail outlet bubble discharge to qualitatively determine the time when the gas breaks through the core, there is a problem of insufficient accuracy, and gas breakthrough can often be monitored at an injection pressure that is significantly higher than the actual value.

[0005] Therefore, to improve the accuracy of the breakthrough pressure experimental device for low-permeability rocks and further enhance the reliability of the breakthrough pressure test results, it is necessary to introduce a gas detection module based on absorption spectroscopy into the test of rock breakthrough pressure, which can monitor the change of gas concentration at the downstream outlet end of the core in real time, visually present the process of gas displacing the liquid inside the core until breakthrough, and thus accurately judge the breakthrough moment and the corresponding breakthrough pressure. A test method and device for breakthrough pressure of low-permeability caprock based on absorption spectroscopy are designed to provide a test means and technical support for solving the technical problem of rapid and accurate measurement of breakthrough pressure of low-permeability caprock and evaluating the sealing performance of the caprock.

[0006] Absorption spectroscopy test methods are widely used in chemical experiments to monitor the generation and participation of gases in reactions. However, there has been no report on the invention and design that can be extended to measure the breakthrough pressure of caprock in geotechnical engineering. Summary of the Invention

[0007] Based on the deficiencies of the above existing technologies, the technical problem to be solved by the present invention is to provide a method for measuring the breakthrough pressure of low-permeability caprock based on absorption spectroscopy sensing, which uses the conventional distributed pressure application method to achieve efficient and accurate measurement of the breakthrough pressure of rocks and monitor the specific leakage amount of gases. At the same time, the entire test device is modular, with convenient installation, systematic management, strong operability, and has the potential for popularization and application.

[0008] Another object of the present invention is to provide a device for measuring the breakthrough pressure of low-permeability caprock based on absorption spectroscopy sensing. It has simple operation, accurate and reliable results, effectively utilizes the absorption characteristics of gases to light, connects the air inlet of the laser reflection cell to the downstream pipeline, and thus realizes the determination of the gas concentration after breakthrough according to the absorption spectroscopy, solving the problems of not being able to accurately capture the gas breakthrough moment and insufficient monitoring of the real-time gas leakage amount in the existing breakthrough pressure test.

[0009] To achieve the above object, the present invention adopts the following technical measures:

[0010] A method for measuring the breakthrough pressure of low-permeability caprock based on absorption spectroscopy sensing, the steps of which are:

[0011] S1: Process the rock into a standard-sized rock sample to be tested, clean it with alcohol and then put it into a drying oven for drying treatment. After stabilization, measure its dry density, height, diameter and volume parameters;

[0012] S2: Obtain the fault rock sample by on-site transformation or obtain the fault surface by using the Brazilian splitting or wire cutting method on the processed cylindrical rock sample;

[0013] S3: Saturate the rock sample with pure deionized water, seal the entire intact rock sample or the combination of fault rock and upper and lower gaskets with heat shrink tubing or silica gel, place it in the core holder and fix it. After installing the core holder module, place it in a constant temperature air bath;

[0014] S4: Connect the laser emitter, laser reflection cell and laser receiver, connect them to the signal conversion software of the computer through the spectrometer;

[0015] S5: Adjust the laser to the absorption peak wavelength corresponding to the injected gas, debug the propagation optical path from laser generation to laser reflection in the reflection cell until accurate reception, and use software and data to calibrate the conversion of the laser absorption spectrum signal to gas concentration to complete the assembly of the absorption spectrum sensing module;

[0016] S6: Arrange liquid barrier films at the inlet and outlet of the reflection cell, connect the inlet of the reflection cell to the downstream pore pressure injection pipeline of the core holder, and then connect other modules to complete the assembly of the entire test device;

[0017] S7: Inject the external confining pressure medium into the confining pressure chamber of the core holder through the confining pressure injection pump and set it to the target confining pressure state for breakthrough testing; then close the back pressure relief valve, disconnect the core holder from the upstream and downstream pipelines, open all upstream, downstream control valves and back pressure valves, and evacuate the upstream and downstream pipelines of the measuring device;

[0018] S8: After waiting for the vacuum to stabilize, connect the core holder to the upstream and downstream pipelines, connect the data monitoring module, record the upstream and downstream pressures and differential pressure data, inject gas upstream through the pore pressure injection pump, and adjust the second upstream control valve and back pressure valve to control the initial pore pressures upstream and downstream of the rock sample to be measured. After the initial upstream and downstream pressures are stable, stop injecting the downstream wetting phase and adjust the back pressure valve;

[0019] S9: Conduct breakthrough pressure testing:

[0020] Adopt the stepwise pressure increase method, that is, gradually increase the upstream gas injection pressure until the breakthrough moment. At this time, the differential pressure between upstream and downstream is the breakthrough pressure; the pore pressure injection pump applies the upstream injection pressure, the first-stage injection pressure P1. After maintaining this pressure for a certain time, if the downstream pressure changes little, continue to gradually increase the upstream injection pressure, that is, the i-th stage pressure P i is larger than P i-1 (i = 2,..., n), and the increase amplitude of each stage is in the range of 0.01 - 1 MPa, and ensure that the injection pressure of each stage is less than the set confining pressure until at P i At a certain moment under the injection pressure, the absorption spectrum sensing module downstream monitors a sudden increase in gas concentration, which is the breakthrough moment. Synchronously record the differential pressure stress P d, which is the breakthrough pressure P at which the gas breaks through the core saturated by the wetting phase b (P b = P d ); During the specific test process, the change and attenuation of the received laser intensity are directly monitored in real time by a spectrometer, and then the corresponding gas concentration is calculated according to the software to directly judge the gas breakthrough;

[0021] S10: After the breakthrough pressure measurement experiment of the rock sample is completed, record the breakthrough pressure P at which the gas breaks through the core b ; Close the pore pressure injection pump, confining pressure injection pump and their corresponding control valves, open all pipelines, open the back pressure relief valve at the end of the device, release the pressure in the test device, and conduct the next measurement.

[0022] In addition, another object of the present invention is to provide a measurement device for the breakthrough pressure of a low-permeability caprock based on absorption spectrum sensing, including a confining pressure injection module, a data monitoring module, a back pressure setting module, an absorption spectrum sensing module, a core holder module and a pore pressure injection module. The confining pressure injection end of the core holder is connected to the confining pressure injection pipeline of the confining pressure injection module to realize the loading of the pressure in the confining pressure chamber of the core holder; the pore pressure injection pump of the pore pressure injection module is connected to the water bath heating box to heat the injected fluid; the upstream pore pressure injection pipeline flows through the intermediate container and is connected to the upstream pore pressure injection pipeline of the core holder module. During the test, as the upstream injection pressure gradually increases, the injected gas can displace the wetting phase in the rock sample to be tested and finally break through the core and enter the downstream pore pressure injection pipeline; according to the size of the core holder module, it is placed in a constant temperature gas bath; the downstream pore pressure injection pipeline of the core holder module is directly connected to the reflection cell air inlet of the laser reflection cell in the absorption spectrum sensing module to monitor the gas leakage at the core outlet end in real time, and the reflection cell air outlet is connected to the back pressure setting module; the upper and lower ends of the laser reflection cell are a laser receiver and a laser transmitter respectively, which are connected to the data monitoring module to realize the conversion from absorption spectrum to gas concentration.

[0023] Furthermore, behind the absorption spectrum sensing module and the downstream pore pressure sensor, a back pressure valve and a flow meter are connected, so as to control the downstream pore pressure to remain stable before breakthrough during the breakthrough pressure test process, and open the back pressure relief valve to release the pressure after the test is completed; the pressure acquisition and processing system and the computer are connected to the upstream pore pressure sensor, downstream pore pressure sensor, confining pressure sensor and differential pressure sensor through the first data line, and the laser transmitter and laser receiver are connected to the spectrometer through the second data line, and the spectrometer is connected to the software in the computer.

[0024] Further, the core holder module includes: a confining pressure chamber, a confining pressure cover, upper and lower pads, a pore pressure fluid pipe of the pads, a heat shrinkable tube or silica gel, an upstream pore pressure injection pipeline, a downstream pore pressure injection pipeline, a confining pressure injection end, an upstream pore pressure control valve, a downstream pore pressure control valve, a differential pressure sensor connected to the upstream and downstream pore pressure pipelines, a control switch valve for connecting the upstream and downstream pore pressure, the upstream pore pressure injection pipeline is connected to the pore pressure injection module, and the downstream pore pressure injection pipeline is connected to the downstream outlet of the core holder module; the injected gas enters the core holder through the upstream pore pressure injection pipeline of the pore pressure injection module, first reaches the upstream pore pressure injection pipeline of the core holder, then displaces the wet phase in the rock sample, and then the gas enters the absorption spectrum sensing module through the downstream pore pressure injection pipeline. The gas migrates upward from the reflection cell inlet of the laser reflection cell to the reflection cell outlet, and finally the back pressure is controlled by the back pressure valve; the confining pressure medium enters the confining pressure chamber of the core holder through the confining pressure injection pipeline to load the confining pressure state of the rock sample.

[0025] Further, the absorption spectrum sensing module includes a laser emitter, a laser receiver, and a laser reflection cell, which are connected to the data monitoring module through data lines for spectral signal analysis through a computer and software system; the data monitoring module also includes a pressure acquisition and processing system, which is connected to the confining pressure sensor, the upstream pore pressure sensor, the downstream pore pressure sensor, and the differential pressure sensor between the upstream and downstream through data lines to realize the processing and analysis of pressure data in combination with the terminal device.

[0026] As described above, compared with the prior art, the method and device for measuring the breakthrough pressure of a low-permeability caprock based on absorption spectrum sensing of the present invention have at least the following advantages:

[0027] 1. The present invention can realize the measurement of the breakthrough pressure of a low-permeability caprock, can complete the test work reliably and efficiently, and comprehensively uses pressure signals and absorption spectrum signals; the present invention innovatively applies absorption spectrum to the breakthrough pressure test, can accurately and effectively judge the moment of breaking through the rock sample, avoids the limited sensitivity and delay of pressure monitoring in the low-permeability caprock test, and provides a good detection and tracking system for breakthrough pressure calculation, breakthrough position and morphological changes of the displacing fluid; at the same time, according to the gas monitoring function, the process and efficiency of gas breaking through the core can be monitored in real time to a certain extent; the measurement structure involved in the present invention is modular, the system is efficient and highly accurate, providing a reliable test means, technical support and theoretical basis for the evaluation method of the sealing performance of low-permeability caprocks.

[0028] 2. The present invention can achieve the measurement of the breakthrough pressure of low-permeability caprock rocks, and can simultaneously characterize and monitor in real time the gas concentration of the breakthrough rocks, so as to accurately determine the breakthrough moment and the corresponding breakthrough pressure. The system includes a confining pressure injection module, a data monitoring module, a backpressure setting module, an absorption spectrum sensing module, a core holder module, and a pore pressure injection module. The present invention is applicable to various pressure and temperature settings, is quick to assemble, stable and reliable, is applicable to the accurate measurement of the caprock breakthrough pressure in geological carbon dioxide storage and underground gas storage, etc., and is also applicable to effectively characterizing and real-time monitoring the process of the wetting phase in the gas-displaced caprock. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings described herein are used to provide a further understanding of the present application and form a part of the present application. The schematic embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation to the present application.

[0030] Figure 1 It is a schematic structural diagram of the measurement device for the breakthrough pressure of low-permeability caprock based on absorption spectrum sensing of the present invention;

[0031] Figure 2 It is a schematic structural diagram of the confining pressure injection module of the present invention;

[0032] Figure 3 It is a schematic structural diagram of the data monitoring module of the present invention;

[0033] Figure 4 It is a schematic structural diagram of the backpressure setting module of the present invention;

[0034] Figure 5 It is a schematic structural diagram of the absorption spectrum sensing module of the present invention;

[0035] Figure 6 It is a schematic structural diagram of the core holder module of the present invention;

[0036] Figure 7 It is a schematic structural diagram of the pore pressure injection module of the present invention;

[0037] Figure 8 It is a schematic diagram of the principle of detecting gas concentration by absorption spectrum;

[0038] Figure 9 It is a schematic test principle diagram of measuring the breakthrough pressure when the brine medium (wetting fluid) in the caprock rock material is broken through by the carbon dioxide medium (non-wetting fluid) by the distributed pressure method.

[0039] Wherein:

[0040] A - Confining pressure injection module: 1 - Confining pressure medium, 2 - External injection pipe, 3 - Confining pressure injection pump, 4 - Confining pressure injection pipeline, 5 - Confining pressure control valve, 6 - Confining pressure sensor;

[0041] B - Data Monitoring Module: 7 - Pressure Acquisition and Processing System, 8 - Computer, 9 - Spectrometer, 10 - First Data Line, 11 - Second Data Line;

[0042] C - Backpressure Setting Module: 12 - Downstream Hole Pressure Sensor, 13 - Backpressure Valve, 14 - Flowmeter, 15 - Backpressure Bleed Valve;

[0043] D - Absorption Spectrum Sensing Module: 16 - Reflecting Cell Inlet, 17 - Liquid Barrier Membrane, 18 - Reflecting Cell Outlet, 19 - Laser Receiver, 20 - Laser Reflecting Cell, 21 - Laser Transmitter;

[0044] E - Core Holder Module: 22 - Upstream Hole Pressure Injection Pipeline, 23 - Downstream Hole Pressure Injection Pipeline, 24 - Confining Pressure Injection End, 25 - Differential Pressure Sensor, 26 - Upstream Hole Pressure Control Valve, 27 - Downstream Hole Pressure Control Valve, 28 - Control Valve for Connecting Upstream and Downstream Hole Pressures, 29 - Stainless Steel Pipeline, 30 - Core Holder, 31 - Rock Sample to be Measured, 32 - Gasket;

[0045] F - Hole Pressure Injection Module: 33 - Water Bath Heating Box, 34 - Gas Cylinder, 35 - Hole Pressure Injection Pump, 36 - Gas Cylinder Control Valve, 37 - First Upstream Control Valve, 38 - Intermediate Container, 39 - Second Upstream Control Valve, 40 - Upstream Hole Pressure Sensor, 41 - Upstream Hole Pressure Injection Pipeline. Specific Embodiment

[0046] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0047] Next, in conjunction with Figures 1 to 9 A measurement method and device for the breakthrough pressure of a low - permeability caprock based on absorption spectrum sensing provided by the present invention will be introduced in detail.

[0048] Embodiment 1:

[0049] The measurement method of the breakthrough pressure of a low - permeability caprock based on absorption spectrum sensing of the present invention is as follows:

[0050] S1: Process the rock into a rock sample 31 to be measured with a standard specification, clean it with alcohol, and then put it into a drying oven for drying treatment (the drying temperature is set at 60 °C). After stabilization, measure its dry density, height, diameter, volume and other parameters;

[0051] S2: The method for obtaining the fault rock sample can be achieved through on-site transformation or by obtaining the fault plane from the processed cylindrical rock sample using methods such as Brazilian splitting or wire cutting.

[0052] S3: Saturate the rock sample with pure deionized water, use heat shrink tubing or silica gel to seal and wrap the entire intact rock sample or the combination of the fault rock and the upper and lower gaskets, and fix it in the core holder 30. After installing the core holder module E, place it in the constant temperature air bath.

[0053] S4: Connect the laser emitter 21, the laser reflection cell 20, and the laser receiver 19, and connect them to the signal conversion software of the computer 8 through the spectrometer 9.

[0054] S5: Adjust the laser to the absorption peak wavelength corresponding to the injected gas, debug the propagation optical path from laser generation to reflection in the laser reflection cell 20 until accurate reception, and use the existing software and data to calibrate the conversion of the laser absorption spectrum signal to the gas concentration to complete the assembly of the absorption spectrum sensing module D.

[0055] S6: Arrange the liquid barrier film 17 at the inlet 16 and the outlet 18 of the reflection cell, connect the inlet 16 of the reflection cell to the downstream pore pressure injection pipeline 23 of the core holder 30, and then connect other modules to complete the assembly of the entire test device.

[0056] S7: Inject the external confining pressure medium 1 (kerosene, deionized water or gas) into the confining pressure chamber of the core holder 30 through the confining pressure injection pump 3 and set it to the target confining pressure state for the breakthrough test; then close the back pressure relief valve 15, disconnect the core holder 30 from the upstream and downstream pipelines, open all the upstream, downstream control valves and the back pressure valve 13, and evacuate the upstream and downstream pipelines of the measuring device.

[0057] S8: After waiting for the vacuum to stabilize, connect the core holder 30 to the upstream and downstream pipelines, connect the data monitoring module B, record the upstream and downstream pressures and the pressure difference data, inject gas (such as CO2, N2, and He gas) into the upstream through the pore pressure injection pump 35, and adjust the second upstream control valve 39 and the back pressure valve 13 to control the initial pore pressures upstream and downstream of the core (the rock sample to be tested 31). After the initial upstream and downstream pressures are stable (ensuring that the pore pressure is less than the confining pressure and the injection end pressure is greater than the threshold set by the back pressure valve 13), stop the injection of the downstream wetting phase and adjust the back pressure valve 13.

[0058] S9: Conduct the breakthrough pressure test:

[0059] The stepwise pressure increase method is adopted, that is, the upstream gas injection pressure is gradually increased until the breakthrough moment. At this time, the pressure difference between the upstream and downstream is the breakthrough pressure. The pore pressure injection pump 35 applies the upstream injection pressure. The first stage injects a relatively small pressure P1 (within the range of 0-10 MPa). After maintaining this pressure for a certain period of time, if the downstream pressure changes little, the upstream injection pressure is continuously increased step by step, that is, the pressure at the i-th stage Pi i is larger than Pi i-1 (i = 2,..., n), and the increase amplitude of each stage is within the range of 0.01-1 MPa, and it is ensured that the injection pressure of each stage is less than the set confining pressure (with a difference of 2-5 MPa), until at Pi i at a certain moment under the injection pressure, the absorption spectrum sensing module D downstream monitors a sudden increase in gas concentration, which is the breakthrough moment. The upstream and downstream differential pressure stress P d of the measuring device is synchronously recorded at this time, which is the breakthrough pressure P b (P b = P d ) at which the gas breaks through the core saturated with the wetting phase. In the specific test process, the change and attenuation of the received laser intensity are directly and real-time monitored through the spectrometer 9, and then the corresponding gas concentration is calculated according to the software to directly judge the gas breakthrough;

[0060] S10: After the breakthrough pressure measurement experiment of the rock sample is completed, the obtained breakthrough pressure P b of the gas breaking through the core is recorded; the pore pressure injection pump 35, the confining pressure injection pump 3 and their corresponding control valves are closed, all pipelines are opened, the back pressure relief valve 15 at the end of the device is opened, and the pressure in the test device is released, and the next measurement can be carried out.

[0061] During the test process, the data monitoring module B simultaneously collects the real-time data of the upstream and downstream pressures, the pressure difference, and the gas concentration of the downstream breakthrough core. Through comprehensive analysis, the process and efficiency of the gas displacing the wetting phase in the pores (fractures) of the core can be characterized in real time to a certain extent.

[0062] The present invention adopts the stepwise pressure increase method to measure the breakthrough pressure of the rock sample, records the upstream and downstream pressures and the pressure difference in real time, monitors the real-time change of the gas concentration of the breakthrough core measured by the absorption spectrum sensing module; according to the sudden change of the downstream gas concentration, the breakthrough moment is determined, the breakthrough pressure is calculated from the corresponding pressure curve, and the efficiency and process of the core breakthrough are further characterized.

[0063] Through the above technical measures, the moment of gas breakthrough in the rock sample can be accurately determined, while the gas leakage volume is monitored and the displacement efficiency is measured. It overcomes the problems of difficulty in determining the breakthrough moment according to the upstream and downstream pressure change curves and the existing delay in the existing breakthrough pressure test device. An absorption spectrum sensing module is innovatively introduced to directly monitor the gas concentration at the outlet of the core, effectively improving the test accuracy and reliability, and providing a good detection and tracking system for determining the gas breakthrough moment, calculating the breakthrough pressure, and the gas breakthrough process, etc.

[0064] Example 2:

[0065] The measuring device for the breakthrough pressure of the low-permeability caprock based on absorption spectrum sensing of the present invention mainly consists of a confining pressure injection module A, a data monitoring module B, a back pressure setting module C, an absorption spectrum sensing module D, a core holder module E, and a pore pressure injection module F. The confining pressure injection end 24 of the core holder 30 is connected to the confining pressure injection pipeline 4 of the confining pressure injection module A to realize the pressure loading in the confining pressure chamber of the core holder 30; the pore pressure injection pump 35 of the pore pressure injection module F is connected to the water bath heating box 33 to heat the injected fluid; the upstream pore pressure injection pipeline 41 flows through the intermediate container 38 and is connected to the upstream pore pressure injection pipeline 22 of the core holder module E. During the test, as the upstream injection pressure gradually increases, the injected gas can displace the wet phase in the core (the rock sample to be tested 31) and finally break through the core and enter the downstream pore pressure injection pipeline 23; according to the size of the core holder module E, it can be placed in a constant temperature gas bath; the downstream pore pressure injection pipeline 23 of the core holder module E is directly connected to the reflection cell gas inlet 16 of the laser reflection cell 20 in the absorption spectrum sensing module D to monitor the gas leakage at the outlet end of the core (the downstream pore pressure injection pipeline 23) in real time, and the reflection cell gas outlet 18 is connected to the back pressure setting module C; the upper and lower ends of the laser reflection cell 20 are respectively a laser receiver 19 and a laser transmitter 21, which are connected to the data monitoring module B to realize the conversion from the absorption spectrum to the gas concentration. After the laser enters the laser reflection cell 20, it is reflected back and forth between the two specially made concave mirrors at both ends, thereby increasing the laser optical path to improve the test accuracy.

[0066] According to the density of the gas and its migration characteristics, adjust the relative heights of the reflection cell gas inlet 16 and the reflection cell gas outlet 18 of the reflection cell 20. For example, in the carbon dioxide breakthrough experiment, the reflection cell gas inlet 16 is set at the lower left (the reflection cell gas outlet 18 is set at the upper right) to measure the gas concentration as accurately as possible; at the same time, liquid barrier films 17 should be installed at the reflection cell gas inlet 16 and the reflection cell gas outlet 18 to prevent errors and damage caused by liquid entering the laser reflection cell 20;

[0067] After the absorption spectrum sensing module D and the downstream pore pressure sensor 12, a back pressure valve 13 and a flow meter 14 are connected to control the downstream pore pressure to remain stable before breakthrough during the breakthrough pressure test process, and the back pressure relief valve 15 is opened to release the pressure after the test; the pressure acquisition and processing system 7 and the computer 8 are connected to the upstream pore pressure sensor 40, the downstream pore pressure sensor 12, the confining pressure sensor 6 and the differential pressure sensor 25 through the first data line 10, and the laser emitter 21 and the laser receiver 19 are connected to the spectrometer 9 through the second data line 11, and the spectrometer 9 is connected to the software in the computer 8.

[0068] The confining pressure injection pipeline 4 passes through the confining pressure control valve 5 and the confining pressure sensor 6 to fill the confining pressure chamber with the confining pressure medium 1 through the confining pressure injection end 24 at the upper part of the core holder 30, so as to reach the set confining pressure state; the pressure acquisition and processing system 7 in the data monitoring module B accesses the signals of the upstream pore pressure sensor 40, the downstream pore pressure sensor 12, the confining pressure sensor 6 and the differential pressure sensor 25 through the first data line 10, and at the same time the spectrometer 9 is connected to the laser emitter 21 and the laser receiver 19 in the absorption spectrum sensing module D through the second data line 11, and is connected to the computer 8 to realize the synchronous real-time monitoring of the pressure data and the downstream gas concentration data.

[0069] The core holder module E includes: a confining pressure chamber, a confining pressure cover, an upper spacer, a lower spacer, a pore pressure fluid pipe of the spacer, a heat shrinkable tube or silica gel, a fluororubber seal ring of the spacer, a core holder sealing bolt, a fluororubber seal ring of the base, an upstream pore pressure injection pipeline 22, a downstream pore pressure injection pipeline 23, a confining pressure injection end 24, an upstream pore pressure control valve 26, a downstream pore pressure control valve 27, a differential pressure sensor 25 connected to the upstream and downstream pore pressure pipelines, and a control switch valve 28 for connecting the upstream and downstream pore pressures. The upstream pore pressure injection pipeline 22 is connected to the pore pressure injection module F, and the downstream pore pressure injection pipeline 23 is connected to the downstream outlet of the core holder module E. The injected gas enters the core holder 30 through the upstream pore pressure injection pipeline 41 of the pore pressure injection module F, first reaches the upstream pore pressure injection pipeline 22 of the core holder 30, then displaces the wet phase in the core sample, and then the gas enters the absorption spectrum sensing module D through the downstream pore pressure injection pipeline 23. The gas migrates upward from the reflection cell inlet 16 of the laser reflection cell 20 to the reflection cell outlet 18, and finally the back pressure is controlled by the back pressure valve 13. The confining pressure medium 1 enters the confining pressure chamber of the core holder 30 through the confining pressure injection pipeline 4 to load the confining pressure state of the core sample. The stainless steel pipeline 29 is connected by a tee, and the two inlets are respectively connected to the reflection cell outlet 18 of the laser reflection cell 20 and the downstream pore pressure control valve, and the outlet is connected to the downstream back pressure setting module C, so as to connect the upstream and downstream of the device and realize the monitoring of the upstream and downstream pressure difference.

[0070] The core holder module E is suitable for multi-scale specimens with a diameter of 25-50mm and a height of 25-150mm. It can test a wide range of specimens including granite, sandstone, shale, mudstone, carbonate rock and artificial cores. The core holder module E is connected to the absorption spectrum sensor module D to achieve accurate judgment of the breakthrough moment and real-time monitoring of gas leakage.

[0071] The rock samples and test pieces to be tested include complete (fault) rock samples, upper pads, and lower pads. For the cut and polished rock samples, directly saturate the rock samples with pure deionized water; for fault rocks, after laying fault mud according to the test plan and consolidating for the designed time, close the fault and temporarily fix the fault rock samples with adhesive tape;

[0072] The absorption spectrum sensing module D includes a laser transmitter 21 (including a modulation module and a collimator), a laser receiver 19, and a laser reflection pool 20 (available on the market), which can be connected to the data monitoring module B via a data line, and spectral signal analysis is performed through a computer and a software system; the data monitoring module B also includes a pressure acquisition and processing system 7, which is connected to the confining pressure sensor 6, the upstream pore pressure sensor 40, the downstream pore pressure sensor 12, and the upper and lower pore pressure differential sensors 25 via a data line, and realizes the processing and analysis of pressure data in combination with the terminal equipment.

[0073] The present invention connects the air inlet 16 of the reflecting pool directly to the downstream pore pressure injection pipeline 23 of the core holder 30, so as to shorten the delay time from gas breakthrough to concentration test as much as possible. The absorption spectrum signal can be converted into a gas concentration value through the analysis software in the computer 8; the upstream and downstream ends of the rock sample 31 to be tested are assembled into a sealed whole by gaskets 32, and the core holder 30 can be placed in an air bath thermostat according to temperature requirements; the pore pressure injection pump 35 is connected to the water bath heating box 33 to inject the gas in the gas cylinder 34 (including the gas cylinder control valve 36) into the rock sample 31 to be tested at a specified temperature, and the state of the injected fluid can be further changed through the intermediate container 38. The first upstream control valve 37 and the second upstream control valve 39 are provided on both sides of the intermediate container 38.

[0074] The present invention innovatively applies absorption spectroscopy to the test of breakthrough pressure, realizes direct detection of breakthrough gas concentration, can accurately and quickly monitor the core breakthrough moment, and simultaneously realizes real-time monitoring of the wetting phase process in the gas-displaced core.

[0075] The working principle of the low permeability cap layer breakthrough pressure measuring device based on absorption spectrum sensing of the present invention is:

[0076] The step-by-step pressurization method is selected according to the application scenario. During the test, the pressure sensor and absorption spectrum sensor module D monitor the process of injecting gas (CO2, He, N2, etc.) to displace the wetting phase (salt water, etc.) in the saturated core in real time, accurately determine the breakthrough moment and measure the corresponding breakthrough pressure.Figure 8 It is a schematic diagram of the principle for detecting gas concentration by absorption spectroscopy. Figure 9 It is a schematic diagram of the test principle for measuring the breakthrough pressure when the brine medium (wetting fluid) in the caprock material is broken through by the carbon dioxide medium (non-wetting fluid) by the distributed pressure method. Where: P i-1 – The upstream gas injection pressure of the previous stage at the breakthrough moment; P o – The initial downstream pressure; △P i – The increase in the upstream gas injection pressure at the i-th stage (i.e., the stage where the breakthrough moment is located in the figure); T - the breakthrough moment of the caprock core; P i – The upstream gas injection pressure at the i-th stage; P b – The minimum critical pressure at which the wetting phase in the test sample 31 is broken through by the gas, that is, the breakthrough pressure. The conversion formula is as follows:

[0077] P b =P i -P0 (1)

[0078] The absorption spectroscopy sensing module adopted in the present invention utilizes the absorption characteristics of gases to light, that is, due to the unique molecular structure of each gas molecule, it has strong absorption for electromagnetic waves of specific wavelengths (such as laser), so that the intensity of the corresponding transmitted light after passing through the gas is significantly reduced (as Figure 8 shown). This characteristic can be expressed by the Beer-Lambert law:

[0079]

[0080] Where, A is the absorbance; I0 is the intensity of the incident light; I t is the intensity of the transmitted light; T is the transmittance; K is the absorption coefficient (related to the gas thickness and concentration); l is the thickness of the gas; c is the concentration of the gas; thus, the gas concentration can be inversely deduced according to the degree of reduction of the light intensity.

[0081] The present invention can realize the measurement of the breakthrough pressure of low-permeability caprocks, can complete the test work reliably and efficiently, and comprehensively utilizes pressure signals and absorption spectroscopy signals; the present invention innovatively applies absorption spectroscopy to the breakthrough pressure test, can accurately and effectively judge the moment of breaking through the rock sample, avoids the limited sensitivity and delay of pressure monitoring in the test of low-permeability caprocks, and provides a good detection and tracking system for breakthrough pressure calculation and gas leakage conditions; at the same time, according to the gas monitoring function, it can monitor the process and efficiency of gas breaking through the core to a certain extent in real time; the measurement structure involved in the present invention is modular, the system is efficient and has high precision, providing a new technical option for the accurate measurement of the breakthrough pressure of low-permeability caprocks.

[0082] The above are only the specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any transformation or replacement that can be understood by those familiar with the technology within the technical scope disclosed by the present invention should be covered within the scope of the present invention.

Claims

1. A method for measuring the breakthrough pressure of a low-permeability caprock based on absorption spectrum sensing, characterized in that The steps are as follows: S1: Process the rock into a test rock sample of standard specifications. After cleaning with alcohol, place it in a drying oven for drying treatment. After stabilization, measure its dry density, height, diameter, and volume parameters. S2: Obtain the fault rock sample. It can be obtained through on-site transformation or by using the Brazilian splitting or wire cutting method on the processed cylindrical rock sample to obtain the fault plane. S3: Saturate the rock sample with pure deionized water. Use a heat shrinkable tube or silicone to seal and wrap the entire intact rock sample or the combination of the fault rock and the upper and lower gaskets. Fix it in a core holder. After installing the core holder module, place it in a constant temperature air bath. S4: Connect the laser emitter, laser reflection cell, and laser receiver. Connect them to the signal conversion software of the computer through a spectrometer. S5: Adjust the laser to the absorption peak wavelength corresponding to the injected gas. Debug the propagation optical path from laser generation to laser reflection in the reflection cell until accurate reception. Use the software and data to calibrate the conversion of the laser absorption spectrum signal to the gas concentration, and complete the assembly of the absorption spectrum sensing module. S6: Arrange liquid barrier membranes at the inlet and outlet of the reflection cell. Connect the inlet of the reflection cell to the downstream pore pressure injection pipeline of the core holder. Then connect other modules to complete the assembly of the entire test device. S7: Inject the external confining pressure medium into the confining pressure chamber of the core holder through a confining pressure injection pump and set it to the target confining pressure state for the breakthrough test. Then close the backpressure relief valve, disconnect the core holder from the upstream and downstream pipelines, open all the upstream, downstream control valves, and the backpressure valve, and evacuate the upstream and downstream pipelines of the measuring device. S8: After waiting for the vacuum to stabilize, connect the core holder to the upstream and downstream pipelines, connect the data monitoring module, record the upstream and downstream pressures and the pressure difference data. Inject gas upstream through the pore pressure injection pump, and adjust the second upstream control valve and the backpressure valve to control the initial pore pressures upstream and downstream of the test rock sample. After the initial upstream and downstream pressures are stable, stop injecting the downstream wetting phase and adjust the backpressure valve. S9: Conduct a breakthrough pressure test: The stepwise pressure application method is adopted, that is, the upstream gas injection pressure is gradually increased until the breakthrough moment, and the pressure difference between the upstream and downstream at this time is the breakthrough pressure; the pore pressure injection pump applies the upstream injection pressure, the first-stage injection pressure P1. After maintaining this pressure for a certain period of time, if the downstream pressure changes little, the upstream injection pressure is continuously increased step by step, that is, the i-th stage pressure P i is larger than P i-1 (i = 2,..., n), and the increase amplitude of each stage is in the range of 0.01 - 1 MPa, and it is ensured that the injection pressure of each stage is less than the set confining pressure until at P i injection pressure, at a certain moment, the sudden increase in gas concentration is monitored by the absorption spectrum sensing module downstream, and this is the breakthrough moment. Synchronously record the upstream and downstream differential pressure stress P d of the measuring device at this time, which is the breakthrough pressure P b (P b = P d ) of the core saturated by the wetting phase broken through by the gas; in the specific test process, the change and attenuation of the received laser intensity are directly monitored in real time by the spectrometer, and then the corresponding gas concentration is calculated according to the software to directly judge the gas breakthrough; S10: After the breakthrough pressure measurement experiment of the rock sample, record the breakthrough pressure P at which the gas breaks through the core. b ; Close the pore pressure injection pump, confining pressure injection pump and their corresponding control valves, open all pipelines, open the back pressure relief valve at the end of the device, release the pressure in the test device, and conduct the next measurement.

2. A measuring device for breakthrough pressure of low-permeability caprock based on absorption spectrum sensing, characterized in that, It includes a confining pressure injection module, a data monitoring module, a back pressure setting module, an absorption spectrum sensing module, a core holder module, and a pore pressure injection module. The confining pressure injection end of the core holder is connected to the confining pressure injection pipeline of the confining pressure injection module to achieve the loading of the pressure in the confining pressure chamber of the core holder. The pore pressure injection pump of the pore pressure injection module is connected to the water bath heating box to heat the injected fluid. The upstream pore pressure injection pipeline flows through the intermediate container and is connected to the upstream pore pressure injection pipeline of the core holder module. During the test, as the upstream injection pressure gradually increases, the injected gas can displace the wet phase in the rock sample to be tested and finally break through the core and enter the downstream pore pressure injection pipeline. According to the size of the core holder module, it is placed in a constant temperature gas bath. The downstream pore pressure injection pipeline of the core holder module is directly connected to the reflection cell air inlet of the laser reflection cell in the absorption spectrum sensing module to monitor the gas leakage at the core outlet end in real time. The reflection cell air outlet is connected to the back pressure setting module. The upper and lower ends of the laser reflection cell are a laser receiver and a laser emitter respectively, which are connected to the data monitoring module to realize the conversion from absorption spectrum to gas concentration.

3. The measuring device for breakthrough pressure of low-permeability caprock based on absorption spectrum sensing according to claim 2, wherein After the absorption spectrum sensing module and the downstream pore pressure sensor, a back pressure valve and a flowmeter are connected, so as to control the downstream pore pressure to remain stable before breakthrough during the breakthrough pressure test process, and the back pressure relief valve is opened to release the pressure after the test ends. The pressure acquisition and processing system and the computer are connected to the upstream pore pressure sensor, the downstream pore pressure sensor, the confining pressure sensor, and the differential pressure sensor through the first data line. The laser emitter and the laser receiver are connected to the spectrometer through the second data line, and the spectrometer is connected to the software in the computer.

4. The measuring device for breakthrough pressure of low-permeability caprock based on absorption spectrum sensing according to claim 3, characterized in that The core holder module includes: a confining pressure chamber, a confining pressure cover, an upper spacer, a lower spacer, the pore pressure fluid pipe of the spacer, a heat shrinkable tube or silica gel, an upstream pore pressure injection pipeline, a downstream pore pressure injection pipeline, a confining pressure injection end, an upstream pore pressure control valve, a downstream pore pressure control valve, a differential pressure sensor connected to the upstream and downstream pore pressure pipelines, a control switch valve for connecting the upstream and downstream pore pressures. The upstream pore pressure injection pipeline is connected to the pore pressure injection module, and the downstream pore pressure injection pipeline is connected to the downstream outlet of the core holder module. The injected gas enters the core holder through the upstream pore pressure injection pipeline of the pore pressure injection module, first reaches the upstream pore pressure injection pipeline of the core holder, then displaces the wet phase in the rock sample, and then the gas enters the absorption spectrum sensing module through the downstream pore pressure injection pipeline. The gas migrates upward from the reflection cell air inlet of the laser reflection cell to the reflection cell air outlet, and finally the back pressure is controlled by the back pressure valve. The confining pressure medium enters the confining pressure chamber of the core holder through the confining pressure injection pipeline to load the confining pressure state of the rock sample.

5. The measuring device for breakthrough pressure of low-permeability caprock based on absorption spectrum sensing according to claim 4, characterized in that The absorption spectrum sensing module includes a laser emitter, a laser receiver, and a laser reflection cell, which are connected to the data monitoring module through a data line and the spectral signal is analyzed through a computer and a software system. The data monitoring module also includes a pressure acquisition and processing system, which is connected to the confining pressure sensor, the upstream pore pressure sensor, the downstream pore pressure sensor, and the differential pressure sensor between the upstream and downstream pores through a data line, and the pressure data is processed and analyzed in combination with the terminal device.

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