Fracturing breakage early warning method for tight reservoir oil and gas well based on core CT scanning

Abstract

The present application relates to the technical field of oil and gas field development engineering combined with digital core technology, and discloses a tight reservoir oil and gas well fracturing breakage early warning method based on core CT scanning, comprising the following steps: obtaining a core sample of a target tight reservoir oil and gas well, performing high-resolution CT scanning on the core sample to obtain three-dimensional CT image data of the core; performing image preprocessing and segmentation processing on the three-dimensional CT image data, identifying the spatial distribution information of minerals, matrix, pores and natural fractures inside the core, and constructing a digital core model based on the spatial distribution information. The present application realizes three-dimensional quantitative characterization of minerals, pores and natural fractures in the tight reservoir core by combining high-resolution core CT scanning with a multi-scale segmentation algorithm, overcomes the deviation problem existing in the fracturing breakage prediction of traditional homogeneous models, and effectively reduces the breakage prediction error.

Description

Technical Field

[0001] This invention relates to the technical field of combining oil and gas field development engineering with digital core technology, and in particular to a method for early warning of fracturing in tight reservoir oil and gas wells based on core CT scanning. Background Technology

[0002] In the development of unconventional tight reservoirs, hydraulic fracturing is one of the core technologies for reservoir stimulation and improving well productivity. However, during fracturing operations, the combined effects of high-pressure fluid injection, stress redistribution, and reservoir heterogeneity can easily lead to problems such as wellbore fracturing or excessive reservoir fracturing. Wellbore fracturing mainly manifests as casing puncture and cement sheath cracking, while excessive reservoir fracturing includes uncontrolled propagation of natural fractures and fracturing fluid channeling. These problems not only cause fracturing operations to fail but also affect the long-term production capacity of oil and gas wells.

[0003] Statistical results show that in horizontal well fracturing operations for shale gas in the Sichuan Basin, approximately 30% of wells experienced casing damage or reservoir channeling due to fracturing failure. Of these, about 80% of the failures stemmed from insufficient characterization of reservoir microstructure, mineral distribution, and other heterogeneous characteristics, as well as inaccurate prediction of stress evolution during fracturing. This indicates that the reservoir's microstructure and its mechanical response during fracturing have a significant impact on fracturing behavior.

[0004] Existing fracturing early warning technologies still have significant shortcomings. On the one hand, traditional methods rely heavily on wellhead pressure monitoring or post-fracturing logging techniques, such as imaging logging, which struggle to capture the propagation of fractures within the reservoir and changes in wellbore stress during fracturing, resulting in a significant lag in early warning. On the other hand, numerical simulation methods typically treat the reservoir as a homogeneous medium, neglecting the influence of core porosity, fractures, and mineral heterogeneity on fracturing behavior, leading to low accuracy in predicting fracturing location and extent. Furthermore, current technologies struggle to effectively couple core micromechanical parameters with fracturing operation parameters, making it impossible to accurately assess the dynamic impact of fracturing flow rate, pressure, and other operation parameters on fracturing risk.

[0005] Therefore, there is an urgent need for a fracturing fracture early warning method that can fully reflect the microscopic heterogeneous characteristics of tight reservoir cores and achieve cross-scale coupled analysis, so as to improve the accuracy and timeliness of fracturing fracture prediction and provide reliable technical support for the safety management and scheme optimization of fracturing operations. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method for early warning of fracturing in tight reservoir oil and gas wells based on core CT scanning.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for early warning of fracturing in tight reservoir oil and gas wells based on core CT scanning includes the following steps: Step 1: Obtain core samples from the target tight reservoir oil and gas well, and perform high-resolution CT scans on the core samples to obtain three-dimensional CT image data of the cores. Step 2: Perform image preprocessing and segmentation on the three-dimensional CT image data to identify the spatial distribution information of minerals, matrix, pores and natural fractures inside the core. Based on the spatial distribution information, construct a digital core model containing the micro-heterogeneous structure of the core characterized by the spatial distribution of minerals, matrix, pores and natural fractures. Embed the initial morphology of the hydraulic fracture in the digital core model to obtain a digital core-initial fracture coupling model. Step 3: Conduct rock mechanics experiments and fracturing physics experiments based on the core samples to obtain rock mechanics parameters and fracturing crack propagation parameters, and calibrate the rock constitutive model and fracture criteria based on the rock mechanics parameters and fracturing crack propagation parameters. Step 4: Based on the digital core-initial fracture coupling model, the calibrated constitutive model and the fracture criterion, construct the hydraulic fracturing-wellbore coupling finite element model, and perform finite element simulation calculations on the hydraulic fracturing process to obtain the fracture propagation characteristics, wellbore mechanical response and reservoir damage distribution during the hydraulic fracturing process. Step 5: Based on the finite element simulation results, establish a fracturing fracturing early warning criterion based on the preset wellbore fracturing index and reservoir fracturing index, and output the corresponding fracturing fracturing early warning level according to the early warning criterion.

[0008] Preferably, the spatial resolution of the high-resolution CT scan in step 1 is 0.4~2 μm, and continuous three-dimensional tomographic images are obtained by scanning the entire length of the core.

[0009] Preferably, the image preprocessing in step 2 includes noise removal, ring artifact correction, and grayscale normalization; the segmentation process includes segmenting the three-dimensional CT image data using a multi-scale threshold segmentation algorithm to extract the spatial distribution information of minerals, pores, and natural cracks.

[0010] Preferably, the rock mechanics experiment in step 3 includes a triaxial compression experiment on the core sample under simulated formation confining pressure, pore pressure and temperature conditions to obtain rock mechanics parameters such as elastic modulus, Poisson's ratio, peak strength, tensile strength, cohesion and internal friction angle. The fracturing physics experiment includes injecting fracturing fluid into core samples under simulated in-situ stress conditions, and obtaining parameters such as critical fracture propagation pressure, fracture length, and fracture width by real-time monitoring of the fracture propagation process.

[0011] Preferably, the rock constitutive model in step 3 is an elastoplastic constitutive model, and damage variables are introduced to characterize the stiffness degradation during rock fracture; the fracture criteria include the maximum tensile stress fracture criterion and the shear fracture criterion, and the fracture threshold parameters are calibrated by the rock mechanical parameters and the hydraulic fracture propagation parameters.

[0012] Preferably, the finite element simulation calculation in step 4 is used to obtain one or more of the following results: The propagation length, width, and propagation rate of the hydraulic fracturing fracture; Equivalent stress and radial displacement of the casing in the wellbore; Location and width of the crack in the cement ring; Spatial distribution of damage variables in reservoir rocks.

[0013] Preferably, the wellbore fracturing indicators in step 5 include casing equivalent stress, casing radial displacement, and cement sheath crack width; the reservoir fracturing indicators include fracturing fracture propagation exceeding limits, fracture channeling risk, and the volume ratio of the reservoir damage zone.

[0014] Preferably, in step 5, the fracturing risk is classified into different early warning levels based on the number of wellbore fracturing indicators and reservoir fracturing indicators that are met.

[0015] The beneficial effects of this invention are as follows: This invention achieves three-dimensional quantitative characterization of minerals, pores, and natural fractures in tight reservoir cores by combining high-resolution core CT scanning with a multi-scale segmentation algorithm. Microstructural information of the core is obtained through 0.4 μm resolution CT scanning, and a digital core model incorporating the core's microscopic heterogeneous structure is constructed. This model accurately reflects the heterogeneous characteristics within tight reservoirs, overcoming the bias issues present in traditional homogeneous models for fracturing prediction and effectively reducing prediction errors by 30%–40%.

[0016] This invention constructs a multi-scale coupled model of "microscopic digital core—macroscopic fracturing fracture—wellbore," directly incorporating rock mechanics parameters obtained from rock mechanics experiments and fracturing fracture propagation parameters obtained from fracturing physics experiments into finite element simulation analysis, realizing the transfer of mechanical response from the mineral pore scale to the fracturing fracture scale. Through this multi-scale coupling method, a unified analysis of fracture propagation and wellbore stress behavior during fracturing can be performed, achieving a fracture location prediction accuracy of over 90%.

[0017] This invention simulates the entire fracturing process using dynamic finite element method (FEM) calculations. It can capture early signs of fracturing before rupture occurs, including sudden increases in fracture propagation rate and casing equivalent stress approaching the yield threshold, thus providing early warning of fracturing failure. Compared to the actual time of rupture, the warning time of this invention can be 5-10 minutes earlier, providing a time window for real-time adjustment of fracturing operation parameters. Furthermore, by establishing a dual-dimensional warning criterion for wellbore fracturing and reservoir fracturing, it can simultaneously identify wellbore fracturing risk and excessive reservoir fracturing risk, avoiding the limitations of single-dimensional warnings.

[0018] This invention is applicable to fracturing operations in tight reservoir oil and gas wells under different burial depths and lithological conditions, with an applicable burial depth range of 1000~8000 m and applicable lithologies including unconventional reservoir types such as shale, tight sandstone, and carbonate rocks. This invention can directly serve the optimization of fracturing schemes and the management of operational safety, for example, guiding the optimized design of construction parameters such as fracturing displacement and pressure, as well as wellbore integrity assessment, providing technical support for safe fracturing operations in unconventional oil and gas wells. By identifying fracturing failure risks in advance and taking corresponding measures, the economic losses caused by fracturing failure can be effectively reduced, with a cost reduction of approximately 500,000 to 2 million yuan per well.

[0019] This invention is an interdisciplinary application of oil and gas field development engineering and digital core technology. By integrating microstructure data obtained from core CT scans, rock mechanics experimental parameters, and fracturing simulation technology, a high-precision and quantitative early warning method for fracturing in tight reservoir oil and gas wells has been established. This method can identify wellbore fracture and excessive reservoir fracture risks in advance, providing a reliable basis for safety management and scheme optimization in fracturing operations. Attached Figure Description

[0020] Figure 1 This is a technical roadmap for the oil and gas well fracturing and fracture early warning method based on core CT scanning, as described in this invention. Figure 2 A three-dimensional structural diagram of the digital core-wellbore-fracture coupling model; Figure 3 This is a finite element structural diagram of a core wellbore before fracturing (before deformation), based on CT scans. Figure 4 This is a finite element structure diagram of the core after fracturing (after stage deformation) based on CT scan. Figure 5 This is a flowchart illustrating the process for determining the early warning level of hydraulic fracturing. Detailed Implementation

[0021] To provide a clearer understanding of the technical features, objectives, and beneficial effects of this invention, the technical solution of this invention is described in detail below, but this should not be construed as limiting the scope of implementation of this invention. Unless otherwise specified, the methods used in this invention are conventional methods in this technical field. In this invention, materials, reagents, or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0022] In one embodiment, the present invention proposes a method for early warning of fracturing in tight reservoir oil and gas wells based on core CT scanning, comprising the following steps: Step 1: Obtain core samples from the target tight reservoir oil and gas well, and perform high-resolution CT scans on the core samples to obtain three-dimensional CT image data of the cores. Step 2: Perform image preprocessing and segmentation on the three-dimensional CT image data to identify the spatial distribution information of minerals, matrix, pores and natural fractures inside the core. Based on the spatial distribution information, construct a digital core model containing the micro-heterogeneous structure of the core characterized by the spatial distribution of minerals, matrix, pores and natural fractures. Embed the initial morphology of the hydraulic fracture in the digital core model to obtain a digital core-initial fracture coupling model. Step 3: Conduct rock mechanics experiments and fracturing physics experiments based on the core samples to obtain rock mechanics parameters and fracturing crack propagation parameters, and calibrate the rock constitutive model and fracture criteria based on the rock mechanics parameters and fracturing crack propagation parameters. Step 4: Based on the digital core-initial fracture coupling model, the calibrated constitutive model and the fracture criterion, construct the hydraulic fracturing-wellbore coupling finite element model, and perform finite element simulation calculations on the hydraulic fracturing process to obtain the fracture propagation characteristics, wellbore mechanical response and reservoir damage distribution during the hydraulic fracturing process. Step 5: Based on the finite element simulation results, establish a fracturing fracturing early warning criterion based on the preset wellbore fracturing index and reservoir fracturing index, and output the corresponding fracturing fracturing early warning level according to the early warning criterion.

[0023] In a preferred embodiment of the present invention, the spatial resolution of the high-resolution CT scan in step 1 is 0.4~2 μm, and continuous three-dimensional tomographic images are obtained by scanning the entire length of the core.

[0024] In a preferred embodiment of the present invention, the image preprocessing in step 2 includes noise removal, ring artifact correction and grayscale normalization; the segmentation process includes segmenting the three-dimensional CT image data using a multi-scale threshold segmentation algorithm to extract the spatial distribution information of minerals, pores and natural cracks.

[0025] As a preferred embodiment of the present invention, the rock mechanics experiment in step 3 includes a triaxial compression experiment on the core sample under simulated formation confining pressure, pore pressure and temperature conditions to obtain rock mechanics parameters such as elastic modulus, Poisson's ratio, peak strength, tensile strength, cohesion and internal friction angle; the fracturing physics experiment includes injecting fracturing fluid into the core sample under simulated in-situ stress conditions, and obtaining parameters such as critical pressure for fracture propagation, fracture length and fracture width by real-time monitoring of the fracture propagation process.

[0026] In a preferred embodiment of the present invention, the rock constitutive model in step 3 is an elastoplastic constitutive model, and damage variables are introduced to characterize the stiffness degradation during rock fracture; the fracture criteria include the maximum tensile stress fracture criterion and the shear fracture criterion, and the fracture threshold parameters are calibrated by the rock mechanical parameters and the hydraulic fracture propagation parameters.

[0027] In a preferred embodiment of the present invention, the finite element simulation calculation in step 4 is used to obtain one or more of the following results: The propagation length, width, and propagation rate of the hydraulic fracturing fracture; Equivalent stress and radial displacement of the casing in the wellbore; Location and width of the crack in the cement ring; Spatial distribution of damage variables in reservoir rocks.

[0028] In a preferred embodiment of the present invention, the wellbore fracturing indicators in step 5 include casing equivalent stress, casing radial displacement, and cement sheath crack width; the reservoir fracturing indicators include fracturing fracture propagation exceeding limits, fracture channeling risk, and reservoir damage zone volume ratio.

[0029] In a preferred embodiment of the present invention, in step 5, the fracturing risk is divided into different warning levels based on the number of wellbore fracturing indicators and reservoir fracturing indicators that are met.

[0030] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0031] Example 1: Obtaining core samples from the target tight reservoir oil and gas well. A full-diameter core sample from the target tight reservoir was selected as the core sample. The core sample was obtained using pressure-holding coring technology to preserve the original pore structure and natural fracture morphology under formation conditions. The obtained core sample underwent pretreatment, including oil and salt washing to remove surface impurities and avoid interference with subsequent scanning processes.

[0032] The core sample was subjected to high-resolution CT scanning. A high-precision CT scanner was used to perform full-length progressive scanning of the core. The scanning spatial resolution was 0.4~2 μm, the scanning voltage was 180~200 kV, and the scanning current was 80~120 μA. The three-dimensional CT image data of the core was obtained by continuous scanning. The three-dimensional CT image data was obtained by generating 2~5 images per millimeter during the scanning process.

[0033] The three-dimensional CT image data undergoes image preprocessing and segmentation. Image preprocessing includes noise removal using Gaussian filtering, elimination of interference from the scanning equipment using a ring artifact correction algorithm, and grayscale normalization to unify the CT values ​​to the range of -1000 to 3000 Hu, thereby enhancing the grayscale differentiation of minerals, pores, and natural fractures. The CT values ​​for quartz minerals range from 1500 to 2000 Hu, and the CT values ​​for pores range from -500 to 0 Hu. Segmentation involves using a multi-scale threshold segmentation algorithm to segment the three-dimensional CT image data. This algorithm combines the Otsu thresholding method and the Watershed algorithm to identify and extract the spatial distribution information of minerals, matrix, pores, and natural fractures within the core. The diameter of the pores ranges from 0.1 to 100 μm, the length of the natural fractures ranges from 5 to 50 mm, and the width ranges from 0.1 to 1 mm.

[0034] Based on the spatial distribution information of the minerals, matrix, pores, and natural fractures, a digital core model is constructed using a three-dimensional reconstruction method. This model includes the microscopic heterogeneous structure of the core, characterized by the spatial distribution of minerals, matrix, pores, and natural fractures. The digital core model is constructed using Avizo software, and its size corresponds proportionally to the actual core sample. In the example, the digital core model has a diameter of 100 mm and a length of 1000 mm. The initial morphology of the hydraulic fracturing fractures is embedded into the digital core model. This initial morphology is determined based on in-situ hydraulic fracturing design parameters, including the fracture initiation position, an initial length of 100 mm, and an initial width of 0.5 mm, thus obtaining a digital core-initial fracture coupling model.

[0035] Rock mechanics and fracturing physics experiments were conducted based on the core samples. In the rock mechanics experiments, plunger samples with a diameter of 25 mm and a length of 50 mm were drilled from the CT-scanned core samples. An RTR-2000 high-temperature, high-pressure triaxial testing system was used to conduct triaxial compression experiments under simulated reservoir depth conditions of confining pressure, pore pressure, and temperature. The confining pressure ranged from 0 to 140 MPa, the pore pressure ranged from 0 to 140 MPa, the temperature ranged from 25 to 150 ℃, and the loading rate was controlled at 0.05 to 0.1 kN / min. The rock mechanics parameters of the core samples, including elastic modulus, Poisson's ratio, peak strength, tensile strength, cohesion, and internal friction angle, were recorded (as shown in Table 1). Simultaneously, acoustic emission monitoring was used to record the initiation and propagation characteristics of microcracks during rock fracturing.

[0036] In the fracturing physics experiment, full-diameter core samples were selected, with a diameter of 100 mm and a length of 300 mm. A true triaxial fracturing system was used to apply simulated in-situ stress to the core samples, with a maximum horizontal stress of 60 MPa, a minimum horizontal stress of 40 MPa, and a vertical stress of 50 MPa. Fracturing fluid was injected into the core samples under different flow rates, ranging from 5 to 20 mL / min. The fracturing fluid was guar gum fracturing fluid with a viscosity of 50–100 mPa·s. Real-time CT scanning was used to record the fracture propagation path, length, width, and branching during the fracturing process, obtaining fracturing propagation parameters, including the critical fracture propagation pressure (55 MPa for fracture and 48 MPa for extension) and fracture conductivity parameters (100 mD·cm for a fracture width of 2 mm).

[0037] Based on the acquired rock mechanical parameters and fracturing crack propagation parameters, the rock constitutive model and fracture criteria are calibrated. The rock constitutive model is an elastoplastic constitutive model, and a damage variable is introduced into the model to characterize the stiffness degradation during rock fracture. The damage variable D is defined as D = 1 - E / E0, where E is the elastic modulus after loading and E0 is the initial elastic modulus. The fracture criteria include the maximum tensile stress fracture criterion and the shear fracture criterion. The fracture threshold parameters are calibrated using the rock mechanical parameters and fracturing crack propagation parameters, where the tensile fracture threshold is 2.5 MPa and the shear fracture threshold is 8 MPa.

[0038] Based on the aforementioned digital core-initial fracture coupling model, calibrated rock constitutive model, and fracture criteria, a fracturing-wellbore coupled finite element model was constructed. This finite element model was discretized using hexahedral elements with a Jacobian determinant of not less than 0.6. Inadequate meshes were optimized through node merging, edge splitting, and surface repair, ultimately generating a finite element model with 10-15 million elements. Mechanical properties were assigned to each component of the model based on rock mechanics experimental data and field parameters (as shown in Table 2). The fracturing fluid was modeled using a linear elasticity, with a density of 1050 kg / m³ and a viscosity of 80 mPa·s. Boundary conditions for the finite element model included applying simulated in-situ stress, applying in-well fracturing pressure to the casing inner wall, constraining normal displacement at the bottom and sides of the model, and applying vertical self-weight stress at the top. Contact properties were also set on the fracturing fracture surface to simulate fracture slip and closure characteristics.

[0039] Finite element method (FEM) simulations were performed on the fracturing process using the OptiStruct solver. The simulation time step was set to 0.1–1 s, simulating the entire process of fracturing fluid injection, fracture propagation, and stress redistribution. Large deformation analysis and contact nonlinear analysis were enabled during the simulation to obtain fracture propagation characteristics, wellbore mechanical response, and reservoir damage distribution. Fracture propagation characteristics included fracture length, fracture width, number of branches, and propagation rate. Wellbore mechanical response included the equivalent stress and radial displacement of the casing. Reservoir damage distribution was represented by the spatial distribution of damage variable D. When damage variable D was greater than 0.6, it was identified as a severely fractured area.

[0040] Based on finite element simulation calculations, the controlling variable method was used to identify the main controlling factors of fracturing failure and analyze the impact of fracturing operation parameters and geological parameters on fracturing failure risk. The fracturing operation parameters included fracturing flow rate, wellbore pressure, and fracturing fluid viscosity, while the geological parameters included the geostress nonuniformity coefficient σH / σh, natural fracture density, and mineral content. The controlled variable analysis results show that when the fracturing flow rate is greater than 15 mL / min and the wellbore pressure is greater than 70 MPa, the fracture propagation rate exceeds 5 mm / s, easily leading to excessive reservoir fracturing. When the geostress nonuniformity coefficient σH / σh is greater than 1.5, the maximum equivalent stress of the casing increases by 20%–30%, significantly increasing the risk of wellbore fracturing. When the natural fracture density is greater than 5 fractures / m and the quartz mineral content is greater than 30%, the fracturing fractures are more likely to propagate along the natural fractures, leading to excessive reservoir stimulation.

[0041] Based on the finite element simulation results and in conjunction with field fracturing cases, a dual-dimensional fracturing fracturing early warning criterion of wellbore fracturing and reservoir fracturing was established, with three levels of warning: yellow, orange, and red (as shown in Table 3). Wellbore fracturing early warning indicators include casing equivalent stress, casing radial displacement, and cement sheath crack width. An early warning is triggered when the casing equivalent stress exceeds 80% of the yield strength (757 MPa for P110 casing); when the casing radial displacement exceeds 5 mm; and when the cement sheath crack width exceeds 0.2 mm. Reservoir fracturing early warning indicators include fracturing fracture extension exceeding limits, fracture channeling risk, and the proportion of reservoir damaged zone volume. An early warning is triggered when the fracturing fracture extension length exceeds the design value by 20% (600 m for a design length of 500 m); when the fracture channels to water or gas layers, the channeling is determined based on numerical simulation of the channeling path; and when the proportion of reservoir damaged zone volume exceeds 15%. By statistically analyzing the number of conditions met for both wellbore fracture early warning indicators and reservoir fracture early warning indicators, the fracturing risk is classified into different early warning levels, and corresponding early warning results are output.

[0042] Based on the finite element simulation results, a fracturing fracturing early warning criterion is established based on preset wellbore fracturing indices and reservoir fracturing indices. The wellbore fracturing indices include casing equivalent stress, casing radial displacement, and cement sheath crack width. The reservoir fracturing indices include fracturing fracture propagation exceeding limits, fracture channeling risk, and reservoir damage zone volume ratio. Based on the number of wellbore fracturing indices and reservoir fracturing indices that are met, the fracturing fracturing risk is divided into different early warning levels.

[0043] Using a Y shale gas well in the Sichuan Basin as an engineering verification example, the well has a burial depth of 3500 m. CT scans of core samples identified a natural fracture density of 6 fractures / m and a quartz content of 28%. Finite element simulation predicted that under fracturing flow rate of 12 mL / min and well pressure of 65 MPa, the maximum equivalent stress of the casing would be 720 MPa, lower than the yield strength of P110 casing, and the fracture propagation length would be 580 m, exceeding the design length of 500 m by 16%, triggering the early warning criterion. During field operations, the fracturing parameters were adjusted, reducing the flow rate to 10 mL / min and controlling the well pressure at 60 MPa. Post-fracturing logging results showed a fracture length of 520 m, no casing rupture, and no reservoir channeling, verifying the effectiveness of the fracturing fracture early warning method.

[0044] The rock mechanics test results were used to obtain the mechanical response parameters of the core samples under different confining pressures. Triaxial compression tests were conducted to record rock mechanics parameters such as elastic modulus, Poisson's ratio, peak strength, tensile strength, cohesion, internal friction angle, and fracture pressure. Table 1 shows the confining pressure conditions and test results corresponding to different rock sample numbers. Table 1

[0045] Based on rock mechanics experimental data and field engineering parameters, corresponding mechanical properties were assigned to each component in the fracturing-wellbore coupled finite element model. These properties included the elastic modulus, Poisson's ratio, strength parameters, and density of the casing, cement sheath, shale matrix, quartz minerals, clay minerals, and fracturing fluid. The range of mechanical property values ​​for each component is shown in Table 2. Table 2

[0046] Based on finite element simulation results and field fracturing cases, a dual-dimensional fracturing fracturing early warning criterion of wellbore fracturing and reservoir fracturing was established. Different early warning levels were set, and the satisfaction status of wellbore fracturing indicators and reservoir fracturing indicators, as well as the risk level and recommended measures, were summarized. The judgment conditions and response measures for different early warning levels are shown in Table 3. Table 3

[0047] Figure 5 The flowchart illustrates the process for determining the early warning level of fracturing, which includes monitoring wellbore and reservoir indicators, determining the early warning level, and implementing response measures.

[0048] In the early warning process, the monitoring indicators strictly correspond to the dual-dimensional indicator system of wellbore fracture and reservoir fracture. The wellbore fracture indicators include three items: casing equivalent stress, casing radial displacement, and cement sheath crack width. The reservoir fracture indicators include three items: fracturing fracture expansion exceeding the limit, fracture flow risk, and the volume ratio of the reservoir damage zone.

[0049] The warning level is determined based on a preset warning level classification rule. By statistically analyzing the number of items that are met by the wellbore fracture index and the reservoir fracture index, the fracturing risk is classified and determined, corresponding to yellow, orange and red warning levels respectively.

[0050] After determining the early warning level, the corresponding response measures are implemented. After the response measures are completed, the system returns to the real-time monitoring stage to continuously update the wellbore and reservoir indicators, thereby forming a complete closed loop for fracturing risk management.

[0051] 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 method for early warning of fracturing in tight reservoir oil and gas wells based on core CT scanning, characterized in that, Includes the following steps: Step 1: Obtain core samples from the target tight reservoir oil and gas well, and perform high-resolution CT scans on the core samples to obtain three-dimensional CT image data of the cores. Step 2: Perform image preprocessing and segmentation on the three-dimensional CT image data to identify the spatial distribution information of minerals, matrix, pores and natural fractures inside the core. Based on the spatial distribution information, construct a digital core model containing the microscopic heterogeneous structure of the core characterized by the spatial distribution of minerals, matrix, pores and natural fractures. Embed the initial morphology of the hydraulic fractures into the digital core model to obtain a digital core-initial fracture coupling model. The segmentation process includes using a multi-scale threshold segmentation algorithm to segment the three-dimensional CT image data to extract the spatial distribution information of minerals, pores and natural fractures. Step 3: Conduct rock mechanics experiments and fracturing physics experiments based on the core samples to obtain rock mechanics parameters and fracturing crack propagation parameters, and calibrate the rock constitutive model and fracture criteria based on the rock mechanics parameters and fracturing crack propagation parameters. Step 4: Based on the digital core-initial fracture coupling model, the calibrated constitutive model and the fracture criterion, construct the hydraulic fracturing-wellbore coupling finite element model, and perform finite element simulation calculations on the hydraulic fracturing process to obtain the fracture propagation characteristics, wellbore mechanical response and reservoir damage distribution during the hydraulic fracturing process. Step 5: Based on the finite element simulation results, establish a fracturing fracturing early warning criterion based on the preset wellbore fracturing index and reservoir fracturing index, and output the corresponding fracturing fracturing early warning level according to the early warning criterion. The rock mechanics experiment described in step 3 includes a triaxial compression test on the core sample under simulated formation confining pressure, pore pressure and temperature conditions to obtain rock mechanics parameters such as elastic modulus, Poisson's ratio, peak strength, tensile strength, cohesion and internal friction angle. The fracturing physics experiment includes injecting fracturing fluid into a core sample under simulated in-situ stress conditions, and obtaining parameters such as critical pressure for fracture propagation, fracture length, and fracture width by real-time monitoring of the fracture propagation process. The rock constitutive model described in step 3 is an elastoplastic constitutive model, and a damage variable is introduced to characterize the stiffness degradation during rock fracture. The fracture criteria include the maximum tensile stress fracture criterion and the shear fracture criterion, and the fracture threshold parameter is calibrated by the rock mechanical parameters and the hydraulic fracture propagation parameters; The wellbore fracturing indicators mentioned in step 5 include casing equivalent stress, casing radial displacement, and cement sheath crack width; the reservoir fracturing indicators include fracturing fracture propagation exceeding limits, fracture channeling risk, and the volume ratio of the reservoir damage zone.

2. The method for early warning of fracturing in tight reservoir oil and gas wells based on core CT scanning according to claim 1, characterized in that, The spatial resolution of the high-resolution CT scan mentioned in step 1 is 0.4~2 μm, and continuous three-dimensional tomographic images are obtained by scanning the entire length of the rock core.

3. The method for early warning of fracturing in tight reservoir oil and gas wells based on core CT scanning according to claim 1, characterized in that, The image preprocessing described in step 2 includes noise removal, ring artifact correction, and grayscale normalization.

4. The method for early warning of fracturing in tight reservoir oil and gas wells based on core CT scanning according to claim 1, characterized in that, The finite element simulation calculations described in step 4 are used to obtain one or more of the following results: The propagation length, width, and propagation rate of the hydraulic fracturing fracture; Equivalent stress and radial displacement of the casing in the wellbore; Location and width of the crack in the cement ring; Spatial distribution of damage variables in reservoir rocks.

5. The method for early warning of fracturing in tight reservoir oil and gas wells based on core CT scanning according to claim 1, characterized in that, In step 5, the fracturing risk is classified into different early warning levels based on the number of wellbore fracturing indicators and reservoir fracturing indicators that are met.

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