Real-time monitoring system and method for high-pressure reservoir high-strength injection-production hole-fracture evolution

Abstract

The present application relates to the technical field of oil and gas field development experiment, and disclose a high-pressure reservoir high-strength injection-production hole-crack evolution real-time monitoring system and method, comprising: true triaxial stress loading module, independent pressure control module, high-strength injection-production module, in-situ dynamic monitoring module and data acquisition and analysis module, each module cooperates to realize the whole process simulation and real-time monitoring of pore-fracture system evolution under high-pressure reservoir high-strength injection-production condition. Through the use of the flow-solid-solid multi-physical field strong nonlinear coupling characteristics caused by high pressure and high speed, the limitations of traditional core displacement experiment in stress loading mode, dynamic monitoring ability and high-speed injection-production simulation are broken through, and through the multi-field coupling experiment device integrating true triaxial stress loading, independent pore pressure control, high-strength injection-production and low-field nuclear magnetic resonance in-situ monitoring, the controllable simulation of real injection-production environment of tight oil reservoir is realized.

Description

Technical Field

[0001] This invention belongs to the field of experimental technology for oil and gas field development, specifically relating to a real-time monitoring system and method for the evolution of high-intensity injection-production well-fracture in high-pressure reservoirs. Background Technology

[0002] my country possesses abundant tight / shale oil reserves, characterized by widespread nano- and micro-sized pores and significant spatial variations in fracture distribution. Currently, the industry commonly employs a long horizontal well volumetric fracturing depletion-based development model. To further enhance the production capacity and recovery rate of tight / shale oil reservoirs, conventional hydraulic fracturing and production enhancement technologies are gradually advancing towards in-depth research involving multi-scale, multi-phase, and multi-field coupling. However, under the core development condition of high-intensity injection and production, systematic research on reservoir structure and multi-field coupled seepage mechanisms remains relatively scarce, becoming a key bottleneck restricting the improvement of development efficiency.

[0003] The economical and effective development of tight oil reservoirs heavily relies on horizontal well drilling and large-scale volumetric fracturing technology, which expands the drainage area by constructing complex fracture networks. The "high-pressure water injection-high-speed oil production" dynamic pressure production mode employed in its development is fundamentally different from the near-equilibrium, low-speed seepage mechanism of conventional oil reservoirs. Under the strong dynamic pressure disturbance caused by high-speed injection and production, the reservoir system exhibits a typical complex process of fluid-solid-chemical multi-field coupling: rapid fluctuations in fluid pressure lead to a redistribution of the effective stress field in the reservoir, thereby inducing elastoplastic deformation of the rock skeleton; fractures and pore structures undergo dynamic evolution under the dual effects of stress and fluid, specifically manifested as fracture opening, expansion, closure, and displacement, as well as pore compression, shear failure, and particle migration; simultaneously, physicochemical reactions may occur between the fluid and rock minerals, further altering the morphology and performance of reservoir seepage channels. This multi-physics, multi-scale nonlinear coupling is the root cause of engineering challenges encountered in field development, such as abnormal fluctuations in injection pressure, rapid decline in production capacity, and failure of fracture conductivity.

[0004] Experimental research is the core approach to obtaining key parameters for reservoir development, revealing seepage mechanisms, and verifying theoretical models. However, existing core displacement devices have significant technical limitations when simulating the multi-field coupling process of high-speed injection and production in tight oil: First, the stress loading method is singular. Most devices use confining pressure holders, which can only apply isotropic equivalent confining pressure and cannot independently control the triaxial principal stress, resulting in a large deviation from the actual triaxial unequal compressive stress state underground. Second, dynamic monitoring capabilities are lacking. The observation of pore structure and fractures relies heavily on destructive methods such as scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), or micro-CT scans before and after the experiment, making it difficult to capture the continuous dynamic changes of the structure during the experiment and to establish a causal relationship between the process and response. Third, the simulation capability for high-speed and high-pressure injection and production is insufficient. Conventional displacement pumps have small displacement and low pressure upper limits, making it difficult to simulate instantaneous large-displacement conditions such as fracturing fluid injection and high-speed injection. Furthermore, the coupling between the injection and production system and the stress control system is poor, making it impossible to accurately control the linkage between pore pressure and confining pressure / axial pressure. These technical bottlenecks make it difficult for existing experimental data to fully support the deepening and development of complex multi-field coupling theory. Developing a multi-field coupling experimental device that can realize high-intensity injection of tight oil under triaxial pressure coupling pore pressure conditions has become an urgent common technical need in this field.

[0005] Against this backdrop, conducting physical simulation and experimental research on multiphase fluid multifield coupled seepage under high-intensity injection and production conditions, developing physical simulation methods for high-intensity injection and production via volumetric fracturing, revealing the fracture evolution characteristics under dynamic loads in different reservoir types, clarifying the fracture evolution laws of high-intensity injection and production in different reservoir types, and the coupling relationship between the seepage field, stress field, and chemical field under different fracture network characteristics after volumetric fracturing and its influence on the seepage field, can provide a solid foundation for subsequent mathematical model establishment and understanding of development laws. This has important scientific and engineering significance for breaking through the technical bottlenecks in the development of tight / shale oil reservoirs and improving development efficiency. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a real-time monitoring system and method for the evolution of high-intensity injection-production well-fracture in high-pressure reservoirs, thereby solving the problems of limited stress loading methods, lack of dynamic monitoring capabilities, and insufficient high-speed high-pressure injection-production simulation capabilities in the background technologies.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a real-time monitoring system for the evolution of pore-fracture system in high-pressure reservoirs with high-intensity injection and production, comprising: a true triaxial stress loading module, an independent pressure control module, a high-intensity injection and production module, an in-situ dynamic monitoring module, and a data acquisition and analysis module. The modules work together to realize the full-process simulation and real-time monitoring of the evolution of the pore-fracture system under high-intensity injection and production conditions in high-pressure reservoirs.

[0008] The true triaxial stress loading module includes a core holder and a triaxial stress loading unit. The core holder adopts an axisymmetric structure design and has built-in core support components, upper and lower end plugs and a multi-stage sealing system. The triaxial stress loading unit includes a confining pressure loading system and an axial stress loading system, which are used to independently adjust the axial stress and confining pressure to construct a true triaxial stress environment of 0-50MPa axial stress and 0-32MPa confining pressure, simulating the in-situ stress state of high-pressure reservoirs. The independent pressure control module includes a pore pressure channel and a pressure control unit, which are used to independently apply pore pressure of 0-50MPa and to achieve linkage control or independent control of pore pressure and injection / production pressure. The high-intensity injection and production module includes an injection channel, a constant speed and constant pressure pump, and a fluid supply unit. The injection channel is formed by drilling axial holes in the rock core and sealing them with pipelines. The constant speed and constant pressure pump includes two control modes: constant pressure and constant flow. The in-situ dynamic monitoring module includes a low-field nuclear magnetic resonance testing unit, which is compatible with the core holder and is used to acquire T2 spectra and perform two-dimensional nuclear magnetic imaging in the X, Y, and Z directions of the core during the injection and production process, thereby capturing the response characteristics of pores and fractures at different scales in real time. The data acquisition and analysis module includes a data acquisition card, a pressure sensor, and a data analysis unit, which is used to acquire injection pressure, pore pressure, and outlet pressure data in real time, and combine them with low-field nuclear magnetic resonance test data to form a quantitative characterization and regular analysis of the pore-crack evolution process.

[0009] Preferably, the core holder includes an outer pressure-bearing shell, a confining pressure loading cavity, a pore pressure channel, and an injection interface; The confining pressure loading cavity uses hydraulic medium to achieve uniform radial loading of the rock core; The upper and lower end plugs are sealed to the core end face using a conical-O-ring composite sealing structure. The multi-stage sealing system uses a combination of O-rings, conical seals, and end face seals to isolate the confining pressure medium from the pore fluid.

[0010] Preferably, the core sample is a cylindrical rock sample with a diameter of 25 mm and a length of 50-80 mm. The axial drilling depth of the rock sample is 25 mm. The drilling is carried out using a tungsten carbide drill bit with an outer diameter of 6 mm. The pipeline is a PEek pipeline with an inner diameter of 2 mm. The borehole and pipeline are sealed together by adhesive bonding.

[0011] Preferably, the low-field nuclear magnetic resonance testing unit is used to identify the signal characteristics of micropores, small pores, medium pores and large pores in the T2 spectrum, and to characterize the dynamic evolution of pore-crack structure by the changes in T2 spectrum morphology and peak distribution.

[0012] Preferably, the data acquisition card has a 12-bit resolution, supports 16-channel single-ended or 8-channel differential input, has a range configuration of 0~10V or ±5V, a system noise of ≤0.4mVrms, the pressure sensor has an IP68 protection rating, a turn-on time of ≤20ms, an analog output resolution of 0.01%FS, and a digital output resolution of 0.05%FS.

[0013] Preferably, it also includes a temperature control module for adjusting the experimental environment temperature to 25-120℃ to simulate the temperature conditions of a high-pressure reservoir.

[0014] A method for real-time monitoring of the evolution of high-intensity injection-production well-fracture in high-pressure reservoirs includes the following steps: S1. Core preparation and pretreatment: High-pressure reservoir rock samples were selected and processed into standard cylindrical cores. After axial drilling, the cores were sealed by PEEK pipeline. The cores were cleaned by circulating toluene and methanol and then vacuum dried at 65℃ and -0.1MPa for 48 hours until constant weight. S2. Basic physical property testing: The porosity of the core was measured by the helium adsorption method, and the gas permeability of the core was tested by the nitrogen method. S3. Core Saturation and Initial Characterization: The dried core was placed in a high-pressure saturation tank and simulated formation water was injected under vacuum pressure. The tank was kept at 20 MPa pressure for 24 hours until saturation. The initial T2 spectrum and three-dimensional nuclear magnetic resonance imaging data of the core were collected using a low-field nuclear magnetic resonance testing unit as reference data. S4. Experimental System Setup and Parameter Setting: The saturated core was loaded into the core holder of the true triaxial stress loading module and connected to the independent pressure control module, high-intensity injection and production module, in-situ dynamic monitoring module, and data acquisition and analysis module; the target values ​​of axial stress, confining pressure, and pore pressure were set, and the system was gradually loaded to the target values ​​and stabilized through the triaxial stress loading unit and pressure control unit. S5. High-intensity injection and production and real-time monitoring: Start the high-intensity injection and production module and carry out high-intensity injection and production operations according to the preset injection rate or pressure mode. The data acquisition and analysis module collects injection pressure, pore pressure and outlet pressure data in real time. During the injection and production process, the low-field nuclear magnetic resonance testing unit is used to periodically collect core T2 spectrum and two-dimensional nuclear magnetic imaging data. S6. Multi-condition comparative test: By adjusting the axial stress loading / unloading mode, injection speed, pore pressure or injection-production cycle number, multiple sets of comparative experiments are carried out, and pressure data and nuclear magnetic resonance data under each condition are continuously collected. S7. Data Processing and Evolution Law Analysis: By comparing the T2 spectrum characteristics, nuclear magnetic resonance imaging results and pressure change data under different injection and production stages and different working conditions, we can quantitatively analyze the dynamic evolution law of pore volume, pore size distribution, fracture propagation and connectivity, and establish the correlation between pore-fracture evolution and injection and production parameters and stress conditions.

[0015] Preferably, in step S4, the stress loading sequence is as follows: first apply an axial stress of 2MPa, then gradually increase the confining pressure to the design value and stabilize it, and finally gradually increase the axial stress to the design value. The pore pressure loading method is as follows: open the core holder outlet, slowly increase the pore pressure until the fluid flows out stably, then close the outlet. When the outlet pressure and the pore pressure inlet pressure are stably equal, the pore pressure setting is completed.

[0016] Preferably, in step S5, the injection rate ranges from 0.01 to 5 ml / min, and the injection-collection cycle includes constant pressure injection to 20 MPa and low pressure backflow at 5 MPa, with the number of cycles not less than 5. Nuclear magnetic resonance (NMR) tests are performed after each pressure stabilization or at the injection-production cycle node to capture the dynamic evolution of pores and fractures in real time.

[0017] Preferably, in step S7, the effective pore volume change is characterized by the T2 spectrum amplitude change, the pore scale evolution is analyzed by the T2 peak migration, the fracture development degree is evaluated by the change in the proportion of long T2 component and two-dimensional nuclear magnetic imaging, and the pore-fracture evolution mechanism and seepage capacity change law under high-intensity injection and production in high-pressure reservoirs are revealed by combining permeability data and pressure response curves. The two-dimensional nuclear magnetic resonance imaging is used to repeatedly scan the same location in the core at set time intervals to acquire a series of images and spectra. By comparing images from different stages and performing differential processing, the location and morphology of newly formed pores and fissures can be identified, thereby quantitatively depicting the evolution of pores and fissures from initiation to expansion and connection.

[0018] Compared with existing technologies, this invention provides a real-time monitoring system and method for high-intensity injection-production well-fracture evolution in high-pressure reservoirs, which has the following beneficial effects: This invention overcomes the limitations of traditional core displacement experiments in terms of stress loading methods, dynamic monitoring capabilities, and high-speed injection-production simulation by utilizing the nonlinear coupling characteristics of fluid-solid-chemical multi-physics fields induced by "high pressure and high speed". Through a multi-field coupling experimental device integrating true triaxial stress loading, independent pore pressure control, high-intensity injection-production, and low-field nuclear magnetic resonance in-situ monitoring, it realizes indoor controllable simulation of the real injection-production environment of tight oil reservoirs.

[0019] At the device level, this invention addresses the key technical challenge of compatibility with NMR testing under high stress loading conditions. It constructs a triaxial core clamping system capable of withstanding high confining pressure, high axial pressure, and high injection pressure. Through multi-stage sealing and the selection of non-magnetic materials, it ensures experimental safety and NMR signal quality. The device allows for independent control of triaxial stress, pore pressure, and injection pressure, and is equipped with a high-precision pressure monitoring and data acquisition system, providing reliable technical support for the quantitative analysis of multi-field coupling processes. Attached Figure Description

[0020] Figure 1 This is a system block diagram of the high-intensity injection-production well-fracture evolution real-time monitoring system for high-pressure reservoirs according to the present invention; Figure 2 This is a schematic diagram of the steps in the high-intensity injection-production well-fracture evolution real-time monitoring method of the present invention. Figure 3 This is a cross-sectional view of the core holder of the present invention; Figure 4 This is a schematic diagram of the structure of the core sample used in the experiment of this invention; Figure 5 This is a flowchart of the high-intensity injection-production experiment of the present invention; Figure 6 This is a schematic diagram comparing the T2 spectra before and after high-intensity injection according to the present invention; Figure 7 This is a schematic diagram comparing the two-dimensional nuclear magnetic resonance imaging results under different injection pressures according to the present invention. Detailed Implementation

[0021] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0022] Example 1: See attached document Figures 1 to 7 The real-time monitoring system for the evolution of pore-fracture system in high-pressure reservoirs with high-intensity injection and production includes: a true triaxial stress loading module, an independent pressure control module, a high-intensity injection and production module, an in-situ dynamic monitoring module, and a data acquisition and analysis module. The modules work together to realize the full-process simulation and real-time monitoring of the evolution of the pore-fracture system under high-intensity injection and production conditions in high-pressure reservoirs. The true triaxial stress loading module includes a core holder and a triaxial stress loading unit. The core holder adopts an axisymmetric structure design and has built-in core support components, upper and lower end plugs and a multi-stage sealing system. The triaxial stress loading unit includes a confining pressure loading system and an axial stress loading system, which are used to independently adjust the axial stress and confining pressure to construct a true triaxial stress environment of 0-50MPa axial stress and 0-32MPa confining pressure, simulating the in-situ stress state of high-pressure reservoirs. The core holder includes an outer pressure-bearing shell, a confining pressure loading cavity, a pore pressure channel, and an injection interface, forming a triaxial stress core holder; The clamp meets four requirements: firstly, it has high mechanical strength and rigidity to withstand a maximum confining pressure and axial pressure of 50MPa; Secondly, it enables independent servo control of triaxial stress. Third, ensure that the core holder can apply individual pore pressure. Fourth, the core holder has an independent injection interface and also meets the testing requirements of low-field NMR.

[0023] The core holder adopts an axisymmetric structural design, such as Figure 3 As shown, it mainly consists of an outer pressure-bearing shell, a confining pressure loading cavity, a core support assembly, upper and lower end plugs, an axial loading mechanism, a pore pressure channel, and a multi-stage sealing system. The core is arranged axially at the center of the holder. The confining pressure loading cavity uses hydraulic medium to achieve uniform radial loading of the rock core; The confining pressure loading system achieves confining pressure loading through an external confining pressure cavity of the clamp, using silicone oil or water as the confining pressure medium. The confining pressure is precisely controlled by an external high-pressure pump, allowing for continuous adjustment and long-term stable maintenance, used to simulate the combined effects of minimum and intermediate principal stresses underground. The confining pressure acts on the outer surface of the core, achieving approximately isotropic radial constraint conditions. The upper and lower end plugs are connected to the core end face using a conical-O-ring composite sealing structure, which ensures load transfer efficiency and prevents pore fluid leakage. The multi-stage sealing system uses a combination of O-rings, conical seals, and end face seals to isolate the confining pressure medium from the pore fluid.

[0024] The axial stress in the axial stress loading system is transmitted to both ends of the core through upper and lower end plugs and is controlled by an independent loading mechanism, which can be adjusted separately based on confining pressure loading. The axial loading and confining pressure loading work together to construct a complete triaxial stress state.

[0025] Under conditions of high confining pressure and high injection pressure, the sealing performance of the device is directly related to experimental safety and data reliability. Therefore, by using a combination of multi-stage O-rings, conical seals, and end-face seals in key parts of the clamp, the confining pressure medium and pore fluid are effectively isolated.

[0026] The independent pressure control module includes a pore pressure channel and a pressure control unit, used to independently apply pore pressure of 0-50MPa and to achieve linkage control or independent regulation of pore pressure and injection-production pressure; by setting an independent pore pressure channel inside the holder, it is used to apply initial pore pressure to the core pores.

[0027] The high-intensity injection and production module includes an injection channel, a constant-speed and constant-pressure pump, and a fluid supply unit. The injection channel is formed by drilling axial holes in the core and sealing them with pipelines. The constant-speed and constant-pressure pump includes two control modes: constant pressure and constant flow. The injection speed can reach 50ml / s, and the injection pressure covers 0-60MPa, simulating the high-intensity injection and production conditions of high-pressure reservoirs. For borehole core samples, a dedicated injection channel is established inside the axial borehole of the core, allowing for the direct application of high-pressure fluid and simulating high-intensity injection-production conditions. The injection system supports both constant pressure and constant flow control modes, with the maximum injection pressure covering the common injection-production pressure range in tight oil reservoirs.

[0028] The core sample is a cylindrical rock sample with a diameter of 25 mm and a length of 50-80 mm. The axial drilling depth of the rock sample is 25 mm. The core sample was taken from a low-permeability sandstone outcrop core in a block of Shengli Oilfield. It is used to simulate the wellhead and surrounding reservoir of a high-intensity injection-production well. The borehole was drilled with a 6 mm outer diameter tungsten carbide drill bit. The pipeline is a 2 mm inner diameter PEEK pipeline, which will be used for low-field nuclear magnetic resonance analysis later. The borehole and pipeline are sealed by adhesive bonding process.

[0029] The in-situ dynamic monitoring module includes a low-field nuclear magnetic resonance (LF-NMR) testing unit, which is compatible with the core holder and is used to acquire T2 spectra and perform two-dimensional nuclear magnetic imaging in the X, Y, and Z directions of the core during the injection and production process, thereby capturing the response characteristics of pores and fractures at different scales in real time. T2 spectral information was acquired from core samples after high-intensity injection using a low-field nuclear magnetic resonance (NMR) testing unit. Continuous acquisition of T2 spectra allows for real-time reflection of the response characteristics of pores and fractures at different scales within the pore structure. Short T2 components correspond to micropores, while long T2 components correspond to macropores or fracture spaces.

[0030] With high-speed injection and production and stress evolution, changes in the morphology and peak distribution of the T2 spectrum can intuitively reveal the dynamic evolution of the pore-fracture structure. Simultaneously, monitoring microfracture changes during high-intensity injection and production using nuclear magnetic resonance imaging provides qualitative and quantitative characterization for high-intensity injection and production in tight reservoirs.

[0031] The low-field nuclear magnetic resonance testing unit is used to identify the signal characteristics of micropores (<1ms), small pores (1-30ms), medium pores (30-100ms), and large pores (>100ms) in the T2 spectrum, and to characterize the dynamic evolution of the pore-crack structure by the changes in T2 spectrum morphology and peak distribution.

[0032] The data acquisition and analysis module includes a data acquisition card, a pressure sensor, and a data analysis unit, which is used to acquire injection pressure, pore pressure, and outlet pressure data in real time, and combine them with low-field nuclear magnetic resonance test data to form a quantitative characterization and regular analysis of the pore-crack evolution process.

[0033] Using low-field nuclear magnetic resonance equipment, based on T2 spectrum feature analysis, image processing and data analysis, we conducted a study on the fracture characteristics generated by high-intensity injection and production at the micrometer scale, providing a theoretical basis for the fracture distribution characteristics of high-intensity injection and production in tight oil and clarifying the injection enhancement mechanism of high-intensity injection and production in tight oil.

[0034] The data acquisition card is a high-precision data acquisition card with a sampling rate of up to 500kS / s. The data acquisition card has a 12-bit resolution, supports 16-channel single-ended or 8-channel differential input, and has a range configuration of 0~10V or ±5V. The system noise is ≤0.4mVrms.

[0035] The pressure sensor has a measurement range of 0-60MPa, an accuracy class of 0.25%FS, an IP68 protection rating, a turn-on time of ≤20ms, an analog output resolution of 0.01%FS, and a digital output resolution of 0.05%FS.

[0036] The present invention also includes a temperature control module for adjusting the experimental environment temperature to 25-120℃ to simulate the temperature conditions of high-pressure reservoirs.

[0037] Five sets of core samples were randomly selected, such as... Figure 4 As shown in the table, the permeability of the core sample is approximately 0.1 mD, and the porosity is approximately 13%. Specific parameters are shown in the table below.

[0038] To effectively simulate the seepage pattern around the injection well during high-intensity water injection in tight reservoirs, this invention's core holder can hold a plunger core with a diameter of 2.5 cm and a length of 5–8 cm. A rubber sleeve applies confining pressure to the core, providing a maximum confining pressure of 60 MPa. By designing 3 mm diameter protruding lines at both the inlet and outlet ends, fluid can be injected into the pores of the rock sample, thereby transforming the linear flow pattern of traditional water-drive experimental cores into a radial flow pattern, effectively simulating the reservoir seepage pattern around the well during high-intensity injection and production. In addition to the core holder, a high-pressure constant-flow pump is used as the power system to achieve a constant-speed injection mode, and a pressure sensor monitors the inlet pressure of the core holder in real time.

[0039] See attached document Figures 2 to 7 A method for real-time monitoring of the evolution of high-intensity injection-production well-fracture in high-pressure reservoirs includes the following steps: S1. Core preparation and pretreatment: High-pressure reservoir rock samples were selected and processed into standard cylindrical cores. After axial drilling, the cores were sealed by PEEK pipeline. The cores were cleaned by circulating toluene (for deasphalting) and methanol (for dehydration and desalination), and then vacuum dried at 65℃ and -0.1MPa for 48 hours until constant weight.

[0040] S2. Basic physical property testing: The porosity of the core was measured by helium adsorption method, and the gas permeability of the core was tested by nitrogen method.

[0041] S3. Core Saturation and Initial Characterization: The dried core was placed in a high-pressure saturation tank and simulated formation water was injected under vacuum pressure. The tank was kept at 20 MPa pressure for 24 hours until saturation. The initial T2 spectrum and three-dimensional NMR imaging data of the core were acquired using a low-field NMR testing unit as reference data. According to the calibration, the T2 peak value between 1-3 ms corresponds to micropores (<0.1 µm), 10-30 ms corresponds to small pores (0.1-1 µm), and signals greater than 100 ms can be attributed to microcracks or extra-large pores (>1 µm).

[0042] S4. Experimental System Setup and Parameter Setting: The saturated core was loaded into the core holder of the true triaxial stress loading module and connected to the independent pressure control module, high-intensity injection and production module, in-situ dynamic monitoring module, and data acquisition and analysis module; the target values ​​of axial stress, confining pressure, and pore pressure were set, and the system was gradually loaded to the target values ​​and stabilized through the triaxial stress loading unit and pressure control unit.

[0043] In this step, the stress loading sequence is as follows: first apply an axial stress of 2 MPa, then gradually increase the confining pressure to the design value and stabilize it, and finally gradually increase the axial stress to the design value. Specifically, first apply an axial pressure of 2 MPa, then apply the confining pressure, slowly increasing the confining pressure to the design value. When the confining pressure stabilizes at the design value, slowly increase the axial pressure to the design value. Observe the confining pressure table and the axial pressure table to ensure that the axial pressure and confining pressure remain stable.

[0044] The pore pressure loading method is as follows: Open the core holder outlet, slowly increase the pore pressure until the fluid flows out stably, then close the outlet. When the outlet pressure and the pore pressure inlet pressure are stably equal, the pore pressure setting is completed. When the pore pressure fluid flows out stably, close the core holder outlet, observe the pressure sensors at the outlet and the pore pressure inlet, and when the two pressure values ​​are stably equal, close the pore pressure inlet to ensure that the pore pressure remains stable.

[0045] S5. High-intensity injection and production and real-time monitoring: Start the high-intensity injection and production module and carry out high-intensity injection and production operations according to the preset injection rate or pressure mode. The injection pressure, pore pressure and outlet pressure data are collected in real time through the data acquisition and analysis module. During the injection and production process, the core T2 spectrum and two-dimensional nuclear magnetic resonance imaging data are collected periodically using the low-field nuclear magnetic resonance testing unit.

[0046] In this step, the injection rate ranges from 0.01 to 5 ml / min, and the injection-collection cycle includes constant pressure injection to 20 MPa and low pressure backflow at 5 MPa, with no less than 5 cycles.

[0047] Nuclear magnetic resonance (NMR) tests are performed after each pressure stabilization or at the injection-production cycle node to capture the dynamic evolution of pores and fractures in real time.

[0048] S6. Multi-condition comparative test: By adjusting the axial stress loading / unloading mode, injection rate, pore pressure or injection-production cycle number, multiple sets of comparative experiments are carried out, and pressure data and nuclear magnetic resonance data under each condition are continuously collected.

[0049] S7. Data Processing and Evolution Law Analysis: By comparing the T2 spectrum characteristics, nuclear magnetic resonance imaging results and pressure change data under different injection and production stages and different working conditions, we can quantitatively analyze the dynamic evolution law of pore volume, pore size distribution, fracture propagation and connectivity, and establish the correlation between pore-fracture evolution and injection and production parameters and stress conditions.

[0050] In this step, the effective pore volume change is characterized by the T2 spectrum amplitude change, the pore scale evolution is analyzed by the T2 peak migration, the fracture development degree is evaluated by the change of the proportion of long T2 component and two-dimensional nuclear magnetic imaging, and the evolution mechanism of pore-fracture and the change law of seepage capacity under high-intensity injection and production in high-pressure reservoirs are revealed by combining permeability data and pressure response curves. The two-dimensional nuclear magnetic resonance imaging is used to repeatedly scan the same location in the core at set time intervals to acquire a series of images and spectra. By comparing images from different stages and performing differential processing, the location and morphology of newly formed pores and fissures can be identified, thereby quantitatively depicting the evolution of pores and fissures from initiation to expansion and connection.

[0051] Reference Figure 7 The comparison of two-dimensional NMR imaging results under different injection pressures is shown (pore pressure 12 MPa, confining pressure 15 MPa, axial pressure 18 MPa). Specific NMR experiments and analysis: To accurately remove fluid signals from the complex pore-fracture network, LF-NMR combined with isotope shielding contrast experiments were introduced. The 'heterotropic blind zone' effect of heavy water (D2O) was utilized to perform a fully blinding pretreatment of the core matrix.

[0052] Based on this, a trace amount of H2O was pre-placed within the fracture as a traceable liquid phase vernier. Under the influence of the injected liquid, the physical shift of the H2O signal peak and the left shift of the T2 spectrum were captured in real time using NMR 1D spatial encoding technology, thereby dynamically quantifying the spatiotemporal evolution of the fracture volume.

[0053] To systematically study the porosity evolution mechanism of high-intensity injection and production in tight oil reservoirs, 11 sets of control experiments were designed, as shown in the table below. All experiments were conducted at room temperature, using simulated formation water as the working fluid.

[0054]

[0055] Before injection and production, the T2 spectrum showed a distinct multi-peak distribution, with the short T2 region (approximately 0.01–0.1 ms) and the medium T2 region (approximately 0.5–5 ms) dominating. This indicates that the core is mainly composed of micropores and small pores, while the contribution of large pores and well-connected channels is limited, and the fluid is mainly controlled by capillary confinement.

[0056] Following high-speed injection and production, the overall spectral amplitude significantly increased, especially the peak value in the medium-to-long T2 region (>1 ms), which broadened towards the long T2 direction, indicating an increase in effective pore volume and an overall increase in pore size. This change reflects that under high pressure differential and high flow velocity, the original micropores and small pores expanded or interconnected, and some of the confined fluid was transformed into mobile fluid. Simultaneously, the long T2 tail was enhanced, indicating an increased contribution from large pores or fracture-type channels, and a significant improvement in pore connectivity.

[0057] From a microscopic perspective, high-speed injection and production significantly increases the fluid shear stress and pressure gradient within the pores. On the one hand, this can scour fine-grained or weakly cemented structures, relieving throat blockage; on the other hand, it induces the initiation and propagation of microcracks, increasing the pore-throat radius and decreasing the pore-throat ratio, thereby weakening capillary confinement. The decrease in the relative proportion of the short T2 zone indicates a reduction in strongly bound fluid, with residual oil mainly migrating to and being utilized in medium and large pores.

[0058] Overall, high-speed injection and production promotes a transformation in the pore structure of tight cores from "micropore-dominated with poor connectivity" to "coexistence of multi-scale pores with enhanced connectivity," which is the key physical basis for improved seepage capacity and reduced residual oil saturation. The next step will be to conduct systematic research on the designed experimental scheme based on preliminary experiments to reveal the pore evolution patterns before and after high-intensity injection and production in tight reservoirs.

[0059] In terms of experimental design, this invention, based on low-permeability tight sandstone cores, designed a systematic comparative experimental scheme covering stress loading / unloading, different injection rates, different pore pressures, and multiple injection-production cycles. It also combined T2 spectroscopy and two-dimensional nuclear magnetic resonance imaging to characterize the dynamic evolution of pore-fracture structures during high-intensity injection-production processes at the microscale.

[0060] Preliminary experimental results show that high-intensity injection and production significantly alters the pore structure of dense cores, causing the T2 spectrum to shift towards the medium-to-long T2 direction. This reflects an increase in effective pore volume, enhanced pore connectivity, and the initiation and propagation of microfractures, verifying the feasibility and sensitivity of the device and experimental scheme. This lays a solid experimental foundation for subsequent systematic elucidation of the pore-fracture evolution mechanism, seepage capacity changes, and increased injection and production mechanisms under high-speed injection and production conditions. It also provides crucial experimental support for the establishment of multi-scale seepage theory and engineering parameter models.

[0061] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A real-time monitoring system for the evolution of high-intensity injection-production well-fracture in high-pressure reservoirs, characterized in that, include: The true triaxial stress loading module, independent pressure control module, high-intensity injection and production module, in-situ dynamic monitoring module, and data acquisition and analysis module work together to realize the full-process simulation and real-time monitoring of the evolution of the pore-fracture system under high-intensity injection and production conditions in high-pressure reservoirs. The true triaxial stress loading module includes a core holder and a triaxial stress loading unit. The core holder adopts an axisymmetric structure design and has built-in core support components, upper and lower end plugs and a multi-stage sealing system. The triaxial stress loading unit includes a confining pressure loading system and an axial stress loading system, which are used to independently adjust the axial stress and confining pressure to construct a true triaxial stress environment of 0-50MPa axial stress and 0-32MPa confining pressure, simulating the in-situ stress state of high-pressure reservoirs. The independent pressure control module includes a pore pressure channel and a pressure control unit, which are used to independently apply pore pressure of 0-50MPa and to achieve linkage control or independent control of pore pressure and injection / production pressure. The high-intensity injection and production module includes an injection channel, a constant speed and constant pressure pump, and a fluid supply unit. The injection channel is formed by drilling axial holes in the rock core and sealing them with pipelines. The constant speed and constant pressure pump includes two control modes: constant pressure and constant flow. The in-situ dynamic monitoring module includes a low-field nuclear magnetic resonance testing unit, which is compatible with the core holder and is used to acquire T2 spectra and perform two-dimensional nuclear magnetic imaging in the X, Y, and Z directions of the core during the injection and production process, thereby capturing the response characteristics of pores and fractures at different scales in real time. The data acquisition and analysis module includes a data acquisition card, a pressure sensor, and a data analysis unit, which is used to acquire injection pressure, pore pressure, and outlet pressure data in real time, and combine them with low-field nuclear magnetic resonance test data to form a quantitative characterization and regular analysis of the pore-crack evolution process.

2. The high-intensity injection-production well-fracture evolution real-time monitoring system for high-pressure reservoirs according to claim 1, characterized in that, The core holder includes an outer pressure-bearing shell, a confining pressure loading cavity, a pore pressure channel, and an injection interface; The confining pressure loading cavity uses hydraulic medium to achieve uniform radial loading of the rock core; The upper and lower end plugs are sealed to the core end face using a conical-O-ring composite sealing structure. The multi-stage sealing system uses a combination of O-rings, conical seals, and end face seals to isolate the confining pressure medium from the pore fluid.

3. The high-intensity injection-production well-fracture evolution real-time monitoring system for high-pressure reservoirs according to claim 1, characterized in that, The core sample is a cylindrical rock sample with a diameter of 25 mm and a length of 50-80 mm. The axial drilling depth of the rock sample is 25 mm. The drilling is carried out using a tungsten carbide drill bit with an outer diameter of 6 mm. The pipeline is a PEek pipeline with an inner diameter of 2 mm. The borehole and pipeline are sealed together using an adhesive bonding process.

4. The high-intensity injection-production well-fracture evolution real-time monitoring system for high-pressure reservoirs according to claim 1, characterized in that, The low-field nuclear magnetic resonance testing unit is used to identify the signal characteristics of micropores, small pores, medium pores and large pores in the T2 spectrum, and to characterize the dynamic evolution of pore-crack structure by the changes in T2 spectrum morphology and peak distribution.

5. The high-intensity injection-production well-fracture evolution real-time monitoring system for high-pressure reservoirs according to claim 1, characterized in that, The data acquisition card has a 12-bit resolution, supports 16-channel single-ended or 8-channel differential input, and has a range configuration of 0~10V or ±5V. The system noise is ≤0.4mVrms. The pressure sensor has an IP68 protection rating, a turn-on time of ≤20ms, an analog output resolution of 0.01%FS, and a digital output resolution of 0.05%FS.

6. The high-intensity injection-production well-fracture evolution real-time monitoring system for high-pressure reservoirs according to claim 1, characterized in that, It also includes a temperature control module, which is used to adjust the experimental environment temperature to 25-120℃ to simulate the temperature conditions of high-pressure reservoirs.

7. The method of the high-intensity injection-production well-fracture evolution real-time monitoring system for high-pressure reservoirs as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Core preparation and pretreatment: High-pressure reservoir rock samples were selected and processed into standard cylindrical cores. After axial drilling, the cores were sealed by PEEK pipeline. The cores were cleaned by circulating toluene and methanol and then vacuum dried at 65℃ and -0.1MPa for 48 hours until constant weight. S2. Basic physical property testing: The porosity of the core was measured by the helium adsorption method, and the gas permeability of the core was tested by the nitrogen method. S3. Core Saturation and Initial Characterization: The dried core was placed in a high-pressure saturation tank and simulated formation water was injected under vacuum pressure. The tank was kept at 20 MPa pressure for 24 hours until saturation. The initial T2 spectrum and three-dimensional nuclear magnetic resonance imaging data of the core were collected using a low-field nuclear magnetic resonance testing unit as reference data. S4. Experimental System Setup and Parameter Setting: The saturated core was loaded into the core holder of the true triaxial stress loading module and connected to the independent pressure control module, high-intensity injection and production module, in-situ dynamic monitoring module, and data acquisition and analysis module; the target values ​​of axial stress, confining pressure, and pore pressure were set, and the system was gradually loaded to the target values ​​and stabilized through the triaxial stress loading unit and pressure control unit. S5. High-intensity injection and real-time monitoring: Start the high-intensity injection module and carry out high-intensity injection and production operations according to the preset injection speed or pressure mode. The data acquisition and analysis module collects injection pressure, pore pressure and outlet pressure data in real time. During the injection and production process, core T2 spectra and two-dimensional nuclear magnetic resonance imaging data are periodically collected using a low-field nuclear magnetic resonance testing unit. S6. Multi-condition comparative test: By adjusting the axial stress loading / unloading mode, injection speed, pore pressure or injection-production cycle number, multiple sets of comparative experiments are carried out, and pressure data and nuclear magnetic resonance data under each condition are continuously collected. S7. Data Processing and Evolution Law Analysis: By comparing the T2 spectrum characteristics, nuclear magnetic resonance imaging results and pressure change data under different injection and production stages and different working conditions, we can quantitatively analyze the dynamic evolution law of pore volume, pore size distribution, fracture propagation and connectivity, and establish the correlation between pore-fracture evolution and injection and production parameters and stress conditions.

8. The method of the high-intensity injection-production well-fracture evolution real-time monitoring system for high-pressure reservoirs according to claim 7, characterized in that, In step S4, the stress loading sequence is as follows: first apply an axial stress of 2MPa, then gradually increase the confining pressure to the design value and stabilize it, and finally gradually increase the axial stress to the design value. The pore pressure loading method is as follows: open the core holder outlet, slowly increase the pore pressure until the fluid flows out stably, then close the outlet. When the outlet pressure and the pore pressure inlet pressure are stably equal, the pore pressure setting is completed.

9. The method of the high-intensity injection-production well-fracture evolution real-time monitoring system for high-pressure reservoirs according to claim 7, characterized in that, In step S5, the injection rate ranges from 0.01 to 5 ml / min, and the injection-collection cycle includes constant pressure injection to 20 MPa and low pressure backflow at 5 MPa, with a cycle of no less than 5 times. Nuclear magnetic resonance (NMR) tests are performed after each pressure stabilization or at the injection-production cycle node to capture the dynamic evolution of pores and fractures in real time.

10. The method of the high-intensity injection-production well-fracture evolution real-time monitoring system for high-pressure reservoirs according to claim 7, characterized in that, In step S7, the effective pore volume change is characterized by the T2 spectrum amplitude change, the pore scale evolution is analyzed by the T2 peak migration, the fracture development degree is evaluated by the change of the proportion of long T2 component and two-dimensional nuclear magnetic imaging, and the evolution mechanism of pore-fracture and the change law of seepage capacity under high-intensity injection and production in high-pressure reservoirs are revealed by combining permeability data and pressure response curves. The two-dimensional nuclear magnetic resonance imaging is used to repeatedly scan the same location in the core at set time intervals to acquire a series of images and spectra. By comparing images from different stages and performing differential processing, the location and morphology of newly formed pores and fissures can be identified, thereby quantitatively depicting the evolution of pores and fissures from initiation to expansion and connection.

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