A method for characterizing damage evolution of hot dry rock fracture network in brittle-ductile transition constitutive

Abstract

This invention relates to the field of geothermal resource development technology, and particularly to a method for characterizing the damage evolution of a hot dry rock fracture network based on a brittle-ductile transformation constitutive model. The method includes experimental data acquisition, transformation criterion construction, coupled constitutive modeling, damage model construction, establishment of a correlation system, and verification and characterization steps. Addressing the problem of existing technologies that employ a purely brittle material assumption and neglect the dynamic transformation of rock mass brittleness and ductility under high temperature and triaxial stress coupling, this invention relies on true triaxial hydraulic fracturing orthogonal experiments to obtain multi-field coupled experimental data, constructs a dynamically updated brittle-ductile transformation threshold criterion, builds a four-field fully coupled numerical model, establishes a two-way correlation system between physical parameters and damage evolution, and completes model calibration and engineering application. In this invention, by constructing a two-way coupled constitutive model of brittle-ductile transformation and damage evolution, combined with dynamic partitioning iteration and dual-parameter inversion, the evolution of rock mass mechanical properties can be accurately reflected, improving the problem of significant deviations between the characterization of the fracture network and actual working conditions.

Description

Technical Field

[0001] This invention relates to the field of geothermal resource development technology, and in particular to a method for characterizing the damage evolution of dry hot rock pressure fracture networks based on brittle-ductile transformation constitutive model. Background Technology

[0002] Hot dry rock, as a clean and renewable geothermal energy source with abundant reserves, wide distribution, and zero carbon emissions, is an important development direction for building a new energy system. Hydraulic fracturing is the core technology for artificially modifying hot dry rock reservoirs and constructing efficient heat exchange reservoirs. The accurate characterization of fracture network damage evolution during fracturing is the core theoretical support for optimizing fracturing schemes, dynamically controlling construction parameters, and quantitatively evaluating the effects of reservoir modification. It directly determines the technical feasibility and economic rationality of hot dry rock geothermal resource development.

[0003] Existing damage characterization techniques for hot dry rock fracture networks are generally based on the linear elastic fracture mechanics framework, simplifying deep hot dry rock masses as homogeneous, purely brittle materials. These techniques can only consider the static strength degradation of the rock mass under fixed temperature and pressure conditions, and therefore fail to consider the continuous dynamic transformation effect of brittleness and ductility of the rock mass during the entire fracturing process under the coupled effects of high temperature, triaxial high stress, and dynamic pore water pressure in deep reservoirs. Furthermore, they have not established a two-way linkage mechanism between the brittleness-ductility transformation process and the damage evolution of the rock mass. This makes it difficult to accurately reproduce the dynamic evolution law of the rock mass's mechanical properties during fracturing, resulting in a systematic deviation between the characterization results of existing models on fracture network initiation, propagation, and damage evolution behavior and actual field conditions. This is the core technical problem that makes it difficult to predict the fracturing effect of hot dry rock and the construction quality of artificial thermal reservoirs below expectations. Summary of the Invention

[0004] To overcome the above shortcomings, this invention provides a method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model. This method aims to improve the existing technology, which is based on the assumption of purely brittle materials, ignores the dynamic transformation effect of rock mass brittleness and ductility, and fails to establish a two-way linkage mechanism between the transformation effect and damage evolution, resulting in significant deviations between the characterization results of the pressure fracture network and actual working conditions.

[0005] This invention provides the following technical solution: a method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model, comprising the following steps:

[0006] S1. Data Acquisition: Conduct a true triaxial hydraulic fracturing orthogonal test covering the target hot dry rock reservoir conditions, and simultaneously collect relevant rock mass test data throughout the entire test process;

[0007] S2. Construction of transformation criteria: Based on the experimental data obtained in S1, the influence of temperature, triaxial principal stress and pore water pressure on the characteristic stress of brittle-ductile transformation of dry hot rocks is analyzed, the boundary of the state interval of brittle-ductile transformation is determined, and the dynamic transformation threshold criterion rule for judging the degree of brittle-ductile transformation of rock mass is established.

[0008] S3. Coupled Constitutive Modeling: Based on the dynamic transformation threshold criterion rule for brittleness and ductility established in S2, the components of the total damage variable of the rock mass are decomposed, a two-way linkage rule between the degree of brittleness and ductility transformation of the rock mass and the damage evolution is established, and a continuous damage mechanics framework is embedded to form a two-way coupled constitutive relationship of the rock mass.

[0009] S4. Damage Model Construction: Based on the rock mass constitutive relationship established in S3, a numerical calculation framework with full coupling of four fields—temperature field, seepage field, stress field, and damage field—is constructed. A fracture propagation criterion matching the degree of brittle-ductile transformation of the rock mass is established, and the numerical model of damage evolution is completed, outputting damage field data.

[0010] S5. Establishment of the correlation system: Based on the standardized test data obtained in S1 and the damage field data output in S4, establish the quantitative correlation between the three-dimensional physical property parameters of hot dry rock and the degree of brittle-ductile transformation of rock mass and the total damage variable, and form the inversion calculation rules for the damage evolution state of rock mass.

[0011] S6. Verification and Characterization: Through indoor experiments and field monitoring data, the numerical model parameters are calibrated and the characterization accuracy is verified to complete the quantitative characterization of the damage evolution of the dry hot rock pressure fracture network.

[0012] By adopting the above technical solutions: First, the dynamic transformation process of brittleness and ductility is incorporated into the fracturing constitutive system to establish a rock mass constitutive relationship that is bidirectionally coupled with brittleness-ductility transformation and damage evolution. Based on the orthogonal test data of full-condition true triaxial hydraulic fracturing, a threshold criterion rule for brittleness-ductility transformation that can be dynamically updated with the fracturing process is constructed to provide quantitative support for rock mass condition determination. Relying on a four-field fully coupled numerical calculation framework, a composite fracture propagation criterion that matches the brittleness-ductility state, and a dynamic partitioning and hierarchical iteration mechanism, accurate characterization of three-dimensional fracture propagation and global damage evolution in hot dry rocks is achieved. By establishing a bidirectional correlation system between physical property parameters and damage state and a dual-parameter collaborative inversion rule, a linkage bridge between theoretical models and field engineering is built. Finally, after indoor test calibration and field data verification, a quantitative characterization method for fracture network damage evolution that can be directly applied to the entire process of hot dry rock fracturing construction is formed.

[0013] Preferably, in step S1, the specific steps for collecting and processing the rock mass-related test data are as follows:

[0014] First, orthogonal test conditions were set to cover the entire operating range of the target hot dry rock reservoir, including temperature, triaxial principal stress, and fracturing fluid injection rate.

[0015] Then, a true triaxial hydraulic fracturing test was carried out, and rock mass stress-strain data, fracturing initiation and propagation pressure data, acoustic emission spatiotemporal distribution data, fracture three-dimensional morphology data, and cross-sectional fractal characteristic data were collected simultaneously throughout the entire test process.

[0016] Finally, all collected data were standardized and outlier removed to obtain standardized experimental data, providing a standardized experimental data source for subsequent steps.

[0017] Preferably, in step S2, the specific steps for constructing the conversion criterion are as follows:

[0018] First, based on the standardized test data obtained from S1, the independent and coupled effects of temperature, triaxial principal stress, and pore water pressure on the characteristic stress of brittle-ductile transition in hot dry rocks were quantitatively analyzed using the response surface methodology.

[0019] Then, based on the variation range of characteristic stress in the brittle-ductile transition, the interval boundaries of the completely brittle state, the brittle-ductile transition state, and the completely ductile state of hot dry rock are determined;

[0020] Finally, a dynamic threshold criterion rule for brittle-ductile transition that can be dynamically updated during the fracturing process and continuously determines the degree of brittle-ductile transition in the rock mass is established. This dynamic threshold criterion rule for brittle-ductile transition is used as the input basis for the bidirectional linkage rule in the subsequent step S3.

[0021] Preferably, in step S3, the specific steps for constructing the bidirectional coupled rock mass constitutive relation are as follows:

[0022] First, based on the dynamic conversion threshold criterion rule for brittleness and ductility established by S2, the total damage variable of the rock mass is decomposed into the brittle damage component generated by fracture tension and the ductile damage component generated by plastic deformation.

[0023] Then, a real-time control rule is established that the higher the degree of brittle-ductile transition of the rock mass, the greater the weight of the ductile damage component and the smaller the weight of the brittle damage component.

[0024] Then, establish a reverse correction rule that the larger the total damage variable of the rock mass, the lower the characteristic stress of brittle-ductile transition;

[0025] Finally, the real-time control rules and the reverse correction rules are embedded into the continuous damage mechanics framework to form a rock mass constitutive relationship that is bidirectionally coupled with brittle-ductile transformation and damage evolution. The rock mass constitutive relationship is used as the core constitutive input for building the numerical model in the subsequent step S4.

[0026] Preferably, in step S4, the specific steps for building the numerical model of damage evolution are as follows:

[0027] First, based on the constitutive relationship of the rock mass established by S3, the thermal conduction control equation of the temperature field, the seepage control equation of the seepage field, the mechanical equilibrium control equation of the stress field, and the evolution control equation of the damage field are simultaneously incorporated to construct a numerical calculation framework that fully couples the four fields of temperature field, seepage field, stress field, and damage field.

[0028] Then, a tension-shear composite fracture propagation criterion matching the real-time brittle-ductile transformation degree of the rock mass was established;

[0029] Finally, a damage evolution numerical model that can simultaneously characterize the dynamic propagation of three-dimensional fractures and the global damage evolution of hot dry rocks is established. The damage evolution numerical model outputs damage field data, which is used as input data for the subsequent step S5 to establish the correlation system.

[0030] Preferably, in step S4, the construction process of the four-field fully coupled numerical computation framework further includes the following steps:

[0031] A dynamic permeability evolution rule was simultaneously established, coupled with the real-time brittle-ductile transformation degree of the rock mass, total damage variable, and triaxial principal stress state.

[0032] The pore water pressure distribution in the seepage field is corrected in real time by the aforementioned dynamic permeability evolution rule;

[0033] The corrected pore water pressure is input in real time into the dynamic brittle-ductility conversion threshold criterion rule established by S2, and the degree of brittle-ductility conversion of rock mass and total damage variables are updated synchronously.

[0034] A fully closed-loop linkage is formed between the seepage field and the rock mass constitutive model, and the updated rock mass brittle-ductile transformation degree and total damage variable are fed back into the fully coupled numerical calculation framework of the four fields.

[0035] Preferably, in step S4, the solution process of the damage evolution numerical model adopts a dynamic partitioning and hierarchical iterative solution mechanism, specifically as follows:

[0036] In each iterative calculation step, the distribution of the rock mass brittle-ductile transformation degree in the entire calculation domain is first updated based on the temperature field, triaxial principal stress field, and pore water pressure field of the current step.

[0037] Then, based on the degree of brittle-ductile transformation of the rock mass, the computational domain is dynamically divided into three independent computational zones: a completely brittle zone, a brittle-ductile transformation zone, and a completely ductile zone.

[0038] Then, the rock mass constitutive relation, tension-shear composite fracture propagation criterion and calculation parameters are established for each zone according to the corresponding S3;

[0039] After simultaneously solving the four-field fully coupled control equations for each partition, the solution results are used as input conditions for updating the distribution of rock mass brittle-ductile transformation degree in the next iteration step.

[0040] Preferably, in step S5, the specific steps for establishing the correlation system between the physical property parameters and damage evolution are as follows:

[0041] First, based on the standardized test data obtained from S1 and the damage field data output from S4, quantitative correlations were established between the triaxial thermal conductivity, triaxial longitudinal wave velocity, and triaxial transverse wave velocity of dry hot rock and the degree of brittle-ductile conversion of the rock mass, as well as the total damage variable.

[0042] Then, based on the quantitative correlation, a calculation rule is established to invert the degree of brittle-ductile transformation of rock mass and the total damage variable through field measured physical property parameters;

[0043] Ultimately, a two-way correlation system between physical property parameters and damage evolution is formed, which can be both forward and inversely modeled. This two-way correlation system is used as the calculation basis for inverting the damage state of rock mass in the field.

[0044] Preferably, in step S5, the process of establishing the bidirectional association system further includes the following steps:

[0045] Based on the spatiotemporal distribution data of acoustic emission collected by S1, a quantitative correlation was established between the degree of brittle-ductile transition of rock mass and the characteristic parameters of acoustic emission of rock mass.

[0046] By combining on-site microseismic monitoring data of the target reservoir with triaxial physical property parameters obtained from well logging, a dual-parameter collaborative inversion rule for the degree of brittle-ductile transformation of the rock mass and the total damage variable is constructed.

[0047] The dual-parameter collaborative inversion rule enables mutual verification between field monitoring data and well logging data, and collaboratively updates the characterization results of rock mass damage evolution.

[0048] Preferably, in step S6, the specific steps of verification and characterization are as follows:

[0049] First, the calculation results of the damage evolution numerical model built in S4 are compared with the experimental data through the indoor true triaxial hydraulic fracturing physical simulation test corresponding to S1, and the core parameters of the model are calibrated.

[0050] Then, the accuracy of the model damage characterization results was verified using microseismic monitoring data and well logging data from in-situ fracturing of the target hot dry rock reservoir.

[0051] Finally, the calibrated numerical model of damage evolution was applied to the quantitative characterization and dynamic analysis of fracture network damage evolution throughout the entire process of hydraulic fracturing in hot dry rock.

[0052] The present invention has the following beneficial effects:

[0053] 1. In this invention, the dynamic transformation process of brittleness and ductility of hot dry rock is incorporated into the hydraulic fracturing constitutive model. Real-time control rules for the degree of brittleness-ductility transformation on brittle and ductile damage components are established, as well as reverse correction rules for the total damage variable on the characteristic stress of brittleness-ductility transformation. This forms a two-way coupled constitutive relationship of the rock mass, breaking through the core limitation of the traditional model that regards hot dry rock as a purely brittle material. This makes the prediction of crack propagation mode more consistent with the actual mechanical behavior under deep high temperature and high stress environment.

[0054] 2. This invention establishes a two-way linkage rule between the degree of brittle-ductile transition and damage evolution, decomposes the total damage variable of the rock mass into brittle damage components and ductile damage components, and implements dynamic weight control. At the same time, it introduces the reverse correction of the characteristic stress of brittle-ductile transition by the total damage variable, realizing a two-way coupled description of the brittle-ductile transition and damage accumulation process, which significantly improves the characterization accuracy of the constitutive model in the brittle-ductile transition stage.

[0055] 3. This invention constructs a fully coupled numerical calculation framework of four fields: temperature field, seepage field, stress field, and damage field. It also establishes a dynamic permeability evolution rule that is linked to the real-time brittle-ductile transformation degree of the rock mass, the total damage variable, and the triaxial principal stress state. This forms a fully closed-loop iterative feedback between the seepage field and the rock mass constitutive model, enabling the numerical model to accurately reflect the real-time impact of fracturing fluid seepage on the mechanical state and damage evolution of the rock mass.

[0056] 4. In the numerical solution process, the present invention adopts a dynamic partitioning and hierarchical iterative solution mechanism. Based on the real-time updated degree of brittle-ductile transformation, the computational domain is dynamically divided into a fully brittle region, a brittle-ductile transformation region, and a fully ductile region, and the corresponding constitutive relations and crack propagation criteria are matched for each region. This effectively solves the technical problem that a single numerical framework cannot simultaneously handle brittle fracture and ductile damage propagation.

[0057] 5. This invention establishes a quantitative correlation between the three-dimensional thermal conductivity of hot dry rock, the three-dimensional longitudinal and transverse wave velocities, the degree of brittle-ductile transformation of the rock mass, and the total damage variable. It also constructs a dual-parameter collaborative inversion rule by combining acoustic emission characteristic parameters. This enables mutual verification of field monitoring data and well logging data and collaborative inversion of damage state, providing a verifiable engineering characterization method for the evolution of fracture network damage during hot dry rock fracturing. Attached Figure Description

[0058] Figure 1 This is a flowchart of a method for characterizing the damage evolution of a dry hot rock pressure fracture network based on a brittle-ductile transformation constitutive model, as proposed in this invention. Detailed Implementation

[0059] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0060] In embodiments of the present invention, a method for characterizing the damage evolution of a dry, hot rock pressure fracture network based on a brittle-ductile transformation constitutive model is provided, such as... Figure 1 As shown, it includes the following steps:

[0061] S1. Data Acquisition: Conduct a true triaxial hydraulic fracturing orthogonal test covering the target hot dry rock reservoir conditions, and simultaneously collect relevant rock mass test data throughout the entire test process;

[0062] Furthermore, the specific steps for collecting and processing rock mass-related test data are as follows:

[0063] First, orthogonal test conditions were set to cover the entire operating range of the target hot dry rock reservoir, including temperature, triaxial principal stress, and fracturing fluid injection rate.

[0064] Then, a true triaxial hydraulic fracturing test was carried out, and rock mass stress-strain data, fracturing initiation and propagation pressure data, acoustic emission spatiotemporal distribution data, fracture three-dimensional morphology data, and cross-sectional fractal characteristic data were collected simultaneously throughout the entire test process.

[0065] Finally, all collected data were standardized and outlier removed to obtain standardized experimental data, providing a standardized experimental data source for subsequent steps.

[0066] Specifically, this step provides a standardized basic data source for the entire technical system. The purpose of this step is to obtain full-cycle hydraulic fracturing test data covering the actual working conditions of the target hot dry rock reservoir, so as to provide basic data support for subsequent criterion construction and constitutive model establishment.

[0067] The specific implementation method is as follows: First, using the actual burial depth, in-situ temperature, triaxial in-situ stress, and on-site fracturing injection parameters of the target hot dry rock reservoir as boundary conditions, a true triaxial hydraulic fracturing orthogonal test is designed, employing test methods conforming to rock mechanics testing specifications. The orthogonal array was used to set the test conditions. The independent variables covered the five core variables of temperature, maximum principal stress, intermediate principal stress, minimum principal stress, and fracturing fluid injection rate in the target reservoir. Each variable was set with four gradient levels to ensure that the test conditions could fully cover the parameter variation range of the entire reservoir fracturing process. The test rock samples were dry hot granite cored from the target reservoir in the field. They were processed into standard cuboid specimens of 50mm×50mm×100mm according to the "Standard for Test Methods of Engineering Rock Mass" GB / T50266. Three parallel specimens were set for each set of conditions to eliminate test dispersion.

[0068] Subsequently, a true triaxial rock mechanics testing system was used to conduct hydraulic fracturing tests. During the tests, high-precision pressure sensors were used to simultaneously collect full-cycle stress-strain data of the rock mass, as well as time-series data of initiation and propagation pressures. Multi-channel acoustic emission sensor arrays were used to simultaneously collect full-parameter data such as the spatiotemporal distribution, ring count, and energy of acoustic emission events. After the tests, industrial CT scans were used to obtain three-dimensional spatial morphology data of the internal fractures of the specimens, and laser confocal microscopy was used to obtain three-dimensional morphology data of the fracture cross-section. The box-counting method was used to calculate the fractal characteristic data of the cross-section morphology.

[0069] Finally, the 3σ criterion was used to remove outlier data points that deviated from the mean by more than three times the standard deviation. All time-series data were synchronously normalized to complete the standardization of the full-cycle test data, resulting in a standardized test dataset. Taking a dry hot rock reservoir at a depth of 4000m as an example, the gradient levels of each test variable were set based on the reservoir's in-situ temperature of 200℃, triaxial in-situ principal stresses of 85MPa / 62MPa / 48MPa, and fracturing fluid injection rate of 8~15mL / min, ensuring that the test conditions were completely matched with the field reservoir conditions.

[0070] The standardized test dataset output in this step will directly serve as the sole input data source for the subsequent construction of the S2 brittleness-ductility conversion criterion and the establishment of the S5 property-damage correlation system.

[0071] S2. Construction of transformation criteria: Based on the experimental data obtained in S1, the influence of temperature, triaxial principal stress and pore water pressure on the characteristic stress of brittle-ductile transformation of dry hot rocks is analyzed, the boundary of the state interval of brittle-ductile transformation is determined, and the dynamic transformation threshold criterion rule for judging the degree of brittle-ductile transformation of rock mass is established.

[0072] Furthermore, the specific steps for constructing the transformation criterion are as follows:

[0073] First, based on the standardized test data obtained from S1, the independent and coupled effects of temperature, triaxial principal stress, and pore water pressure on the characteristic stress of brittle-ductile transition in hot dry rocks were quantitatively analyzed using the response surface methodology.

[0074] Then, based on the variation range of characteristic stress in the brittle-ductile transition, the interval boundaries of the completely brittle state, the brittle-ductile transition state, and the completely ductile state of hot dry rock are determined;

[0075] Finally, a dynamic threshold criterion rule for brittle-ductile transition that can be dynamically updated during the fracturing process and continuously determines the degree of brittle-ductile transition in the rock mass is established. This dynamic threshold criterion rule for brittle-ductile transition is used as the input basis for the bidirectional linkage rule in the subsequent step S3.

[0076] Specifically, this step establishes core state determination rules for the entire technical system. The purpose of this step is to quantify the influence of multi-field coupling factors on the brittle-ductile transformation of dry hot rocks, establish a continuous determination method for brittle-ductile state that can be dynamically updated with the fracturing process, and provide core criteria for subsequent bidirectional coupling modeling.

[0077] The specific implementation method is as follows: Based on the standardized test dataset obtained by S1, the full stress-strain curve and the corresponding volumetric strain curve under each test condition are first processed. The curvature change of the volumetric strain curve is calculated using the five-point difference method. The axial deviatoric stress corresponding to the extreme change point of the volumetric strain curvature is extracted as the brittle-ductile transformation characteristic stress of the dry hot rock under that condition. Using the brittle-ductile transformation characteristic stress as the response value and temperature, triaxial principal stress, and pore water pressure as independent variables, a multivariate nonlinear regression analysis is carried out using the Box-Behnken response surface method. The significance of the regression model is tested through analysis of variance to quantify the independent and coupled effects of each variable on the brittle-ductile transformation characteristic stress. A multi-factor coupled explicit calculation model for the brittle-ductile transformation characteristic stress is established, and the formula is as follows:

[0078]

[0079] In the formula: The characteristic stress of brittle-ductile transformation under current working conditions in hot dry rock is given in MPa; σ cd0 The characteristic stress of brittle-ductile transformation of hot dry rock under standard conditions of normal temperature and pressure is expressed in MPa and is directly obtained from the benchmark test of S1; T is the current absolute temperature of the rock mass in K; T0 is the benchmark value of normal temperature, which is fixed at 298.15 K. α represents the current maximum principal stress, intermediate principal stress, and minimum principal stress of the rock mass, respectively, in MPa; P represents the current pore water pressure of the rock mass, in MPa; α is the temperature influence coefficient, β is the triaxial principal stress coupling influence coefficient, and γ is the pore water pressure influence coefficient, all obtained by response surface regression fitting, and are dimensionless.

[0080] Based on the real-time brittle-ductile transition characteristic stress obtained from the above model, combined with the current deviatoric stress σ of the rock mass... dev=σ1−σ3, determine the boundaries of the three state intervals for the brittle-ductile transition of hot dry rocks, establish a continuous brittle-ductile transition degree calculation formula that can be dynamically updated with the fracturing process, and realize the real-time quantitative determination of the brittle-ductile state of the rock mass. The formula is as follows:

[0081]

[0082] In the formula: D cd The current brittle-ductile transformation degree of the rock mass is dimensionless and ranges from 0 to 1; D cd =0 corresponds to a completely brittle state of the rock mass, D cd =1 corresponds to the rock mass being in a fully ductile state, 0 <D cd <1 corresponds to the transition state of the rock mass from brittle to ductile.

[0083] The brittle-ductility dynamic transition threshold criterion rule established in this step can be updated in real time with the dynamic changes of stress field, temperature field, and seepage field during fracturing, accurately quantifying the current brittle-ductility state of the rock mass and outputting the brittle-ductility transition degree D. cd This will directly serve as the core input parameter for the subsequent S3 bidirectional coupled structural model and the S4 numerical model fracture propagation criterion. Taking the aforementioned 4000m deep reservoir condition as an example, when fracturing fluid injection causes an increase in pore water pressure P, σ can be calculated in real time using the above model. cd The dynamic changes, and update D synchronously. cd The value of is used to continuously track the brittle-ductile state of the rock mass throughout the entire fracturing process.

[0084] S3. Coupled Constitutive Modeling: Based on the dynamic transformation threshold criterion rule for brittleness and ductility established in S2, the components of the total damage variable of the rock mass are decomposed, a two-way linkage rule between the degree of brittleness and ductility transformation of the rock mass and the damage evolution is established, and a continuous damage mechanics framework is embedded to form a two-way coupled constitutive relationship of the rock mass.

[0085] Furthermore, the specific steps for constructing the two-way coupled constitutive relation of the rock mass are as follows:

[0086] First, based on the dynamic conversion threshold criterion rule for brittleness and ductility established by S2, the total damage variable of the rock mass is decomposed into the brittle damage component generated by fracture tension and the ductile damage component generated by plastic deformation.

[0087] Then, real-time control rules were established for the rock mass with a higher degree of brittle-ductile transition, a larger weight of ductile damage component, and a smaller weight of brittle damage component.

[0088] Then, establish a reverse correction rule that the larger the total damage variable of the rock mass, the lower the characteristic stress of brittle-ductile transition;

[0089] Finally, the real-time control rules and reverse correction rules are embedded into the continuous damage mechanics framework to form a rock mass constitutive relationship that is bidirectionally coupled with brittle-ductile transformation and damage evolution. The rock mass constitutive relationship is used as the core constitutive input for building the numerical model in the subsequent step S4.

[0090] Specifically, this step establishes the core mechanical constitutive model for the entire technical system. The purpose of this step is to embed the brittle-ductile dynamic transformation process into the damage mechanics framework, establish a two-way coupled constitutive relationship between brittle-ductile transformation and damage evolution, and break through the limitations of the traditional pure brittle damage model.

[0091] The specific implementation method is as follows: Based on the dynamic transformation threshold criterion rule of brittleness and ductility established by S2, and with continuous damage mechanics as the theoretical framework, the total damage variable of the rock mass is decomposed into brittle damage component generated by fracture tension and ductile damage component generated by plastic deformation. Real-time control rules of brittleness and ductility transformation degree on the two types of damage components are established, as well as reverse correction rules of the total damage variable on the characteristic stress of brittleness and ductility transformation. Finally, a rock mass constitutive relationship with bidirectional coupling between brittleness and ductility transformation and damage evolution is formed.

[0092] Based on the effective stress principle of continuous damage mechanics, and considering the anisotropic characteristics of rock mass damage, the effective stress tensor of the rock mass can be expressed as:

[0093]

[0094] In the formula: σ′ is the effective stress tensor of the rock mass, in MPa; σ is the Cauchy stress tensor of the rock mass, in MPa; I is the second-order unit tensor, dimensionless; D is the second-order symmetric damage tensor, dimensionless, whose principal value corresponds to the total damage variable in the three principal stress directions, and its value ranges from 0 to 1.

[0095] Real-time brittle-ductile transition degree D based on S2 output cd A weighted control formula for the total damage variable is established to achieve positive control of the degree of brittleness-ductility transition on damage evolution. The formula is as follows:

[0096] ;

[0097] In the formula: D represents the current total damage variable of the rock mass, which is dimensionless, and is the principal value of the damage tensor corresponding to the principal stress direction; D b The brittle damage component is dimensionless and arises from tensile fracture. It is calculated based on the normalized cumulative ringing count of acoustic emission acquired by S1. (D) p The ductile damage component is dimensionless and is generated by the accumulation of dislocations and the evolution of micropores during plastic deformation. It is calculated based on the normalized plastic dissipation energy density obtained from the S1 test.

[0098] A reverse correction rule for the total damage variable on the brittle-ductile transition characteristic stress is established simultaneously. The corrected formula for calculating the brittle-ductile transition characteristic stress is as follows:

[0099]

[0100] In the formula: The brittle-ductile transition characteristic stress after damage correction, in units of The damage influence coefficient is dimensionless and calibrated by fitting the experimental data of S1; the corrected... Feeding into the brittle-ductile transition degree calculation formula of S2, D is updated synchronously. cd The value of is used to form a two-way closed-loop linkage between damage evolution and brittleness-ductility transition degree regulation of damage evolution and damage evolution correction of brittleness-ductility transition threshold.

[0101] The two-way coupled constitutive relation established in this step is the first to realize the full-process linkage between the dynamic transformation of brittleness and ductility and the evolution of rock mass damage. The output two-way coupled constitutive relation will be directly used as the core mechanical constitutive input for the subsequent construction of the S4 four-field coupled numerical model.

[0102] S4. Damage Model Construction: Based on the rock mass constitutive relationship established in S3, a numerical calculation framework with full coupling of four fields—temperature field, seepage field, stress field, and damage field—is constructed. A fracture propagation criterion matching the degree of brittle-ductile transformation of the rock mass is established, and the numerical model of damage evolution is completed, outputting damage field data.

[0103] Furthermore, the specific steps for building the numerical model of damage evolution are as follows:

[0104] First, based on the constitutive relationship of the rock mass established by S3, the thermal conduction control equation of the temperature field, the seepage control equation of the seepage field, the mechanical equilibrium control equation of the stress field, and the evolution control equation of the damage field are simultaneously incorporated to construct a numerical calculation framework that fully couples the four fields of temperature field, seepage field, stress field, and damage field.

[0105] Then, a tension-shear composite fracture propagation criterion matching the real-time brittle-ductile transformation degree of the rock mass was established;

[0106] Finally, a damage evolution numerical model that can simultaneously characterize the dynamic propagation of three-dimensional fractures and the global damage evolution of hot dry rocks was built. The damage evolution numerical model outputs damage field data, which is used as input data for the subsequent step S5 to establish the correlation system.

[0107] Furthermore, the construction of the four-field fully coupled numerical computation framework also includes the following steps:

[0108] A dynamic permeability evolution rule was simultaneously established, coupled with the real-time brittle-ductile transformation degree of the rock mass, total damage variable, and triaxial principal stress state.

[0109] The pore water pressure distribution in the seepage field is corrected in real time by using dynamic permeability evolution rules;

[0110] The corrected pore water pressure is input in real time into the dynamic brittle-ductility conversion threshold criterion rule established by S2, and the degree of brittle-ductility conversion of rock mass and total damage variables are updated synchronously.

[0111] A fully closed-loop linkage is formed between the seepage field and the rock mass constitutive model, and the updated rock mass brittle-ductile transformation degree and total damage variable are fed back into the four-field fully coupled numerical calculation framework;

[0112] Furthermore, the solution process of the damage evolution numerical model adopts a dynamic partitioning and hierarchical iterative solution mechanism, with the following specific steps:

[0113] In each iterative calculation step, the distribution of the rock mass brittle-ductile transformation degree in the entire calculation domain is first updated based on the temperature field, triaxial principal stress field, and pore water pressure field of the current step.

[0114] Then, based on the degree of brittle-ductile transformation of the rock mass, the computational domain is dynamically divided into three independent computational zones: a completely brittle zone, a brittle-ductile transformation zone, and a completely ductile zone.

[0115] Then, the rock mass constitutive relation, tension-shear composite fracture propagation criterion and calculation parameters are established for each zone according to the corresponding S3;

[0116] After simultaneously solving the four-field fully coupled control equations for each partition, the solution results are used as input conditions for updating the distribution of rock mass brittle-ductile transformation degree in the next iteration step.

[0117] Specifically, this step provides a numerical implementation carrier for the entire technical system. The purpose of implementation is to embed the bidirectional coupled constitutive model into the multi-field coupled numerical framework, establish a numerical model that can accurately characterize the three-dimensional fracture propagation and damage evolution of hot dry rock fracturing, and realize the computability of theoretical methods.

[0118] The specific implementation method is as follows: Based on the two-way coupled rock mass constitutive relation established in S3, the finite element numerical method is used to construct a fully coupled numerical calculation framework of four fields: temperature field, seepage field, stress field, and damage field. The control equations of each physical field are incorporated into the framework: the temperature field adopts the Fourier heat conduction control equation, which simultaneously considers the dynamic influence of rock mass damage and temperature on thermal conductivity; the seepage field adopts the Darcy seepage control equation, which simultaneously considers the dynamic influence of rock mass damage and brittle-ductile state on permeability; the stress field adopts the linear elastic mechanical equilibrium control equation, which is embedded in the two-way coupled damage constitutive relation established in S3; and the damage field adopts the aforementioned damage evolution equation, which is linked in real time with the brittle-ductile conversion degree established in S2.

[0119] Based on the S2 brittle-ductile transition degree interval division, a tension-shear composite fracture propagation criterion matching the real-time brittle-ductile transition degree of the rock mass is established: when D cd When 0.2 < 0.2, the maximum tensile stress criterion is used to determine crack initiation and propagation under tension; when 0.2 ≤ D cdWhen D ≤ 0.8, the tension-shear composite strength criterion is adopted, simultaneously considering both tension and shear failure modes; when D cd When the value is greater than 0.8, the Drucker-Prager yield criterion is used to determine the shear slip propagation of the crack.

[0120] During the construction of the numerical calculation framework, a dynamic permeability evolution rule was simultaneously established, coupled with the real-time brittle-ductile transition degree of the rock mass, total damage variable, and triaxial principal stress state. The formula is as follows:

[0121] ;

[0122] In the formula: k is the current permeability of the rock mass, in mD; k0 is the initial permeability of the rock mass under undamaged conditions, in mD, obtained from the experimental data of S1; ξ is the damage influence coefficient and ζ is the brittle-ductile conversion degree influence coefficient, which are dimensionless and are both calibrated by fitting experimental data.

[0123] The dynamic permeability evolution rule is used to correct the pore water pressure distribution of the seepage field in real time. The corrected pore water pressure is then input into the dynamic brittle-ductile transformation threshold criterion rule of S2 in real time, and the brittle-ductile transformation degree and total damage variable of the rock mass are updated simultaneously, forming a closed-loop linkage between the seepage field, constitutive model and damage field.

[0124] The numerical model solution process employs a dynamic partitioned hierarchical iterative solution mechanism. The specific steps are as follows: In each iterative calculation step, firstly, based on the temperature field, triaxial principal stress field, and pore water pressure field converged in the current step, the distribution of the rock mass brittle-ductile transition degree across the entire computational domain is updated; then, according to the brittle-ductile transition degree, the computational domain is dynamically divided into three independent computational partitions: a completely brittle zone, a brittle-ductile transition zone, and a completely ductile zone; next, corresponding two-way coupled rock mass constitutive relations, fracture propagation composite criteria, and computational parameters are matched for each partition; simultaneously, the four-field fully coupled governing equations for each partition are solved. When the global stress residual is less than... Temperature residual less than When the current iteration step converges, the solution is determined to be converged. The converged solution is then used as the input condition for updating the rock mass brittle-ductile transformation degree distribution in the next iteration step.

[0125] The numerical model built in this step can accurately simulate the three-dimensional dynamic propagation process of fracturing fractures in hot dry rock under different brittle-ductile states, achieving real-time characterization of the entire damage evolution. The output damage field data will be directly used as input data for the subsequent establishment of the S5 property-damage correlation system. Taking the aforementioned numerical model of a 4000m deep reservoir as an example, during the iterative calculation of fracturing fluid injection, when the calculated D of the near-wellbore rock mass... cdWhen the value is increased to 0.7, the model will automatically match the composite criterion and corresponding constitutive parameters for the region, and synchronously update the permeability and pore water pressure of the region to achieve continuous simulation of the fracture propagation process.

[0126] S5. Establishment of the correlation system: Based on the standardized test data obtained in S1 and the damage field data output in S4, establish the quantitative correlation between the three-dimensional physical property parameters of hot dry rock and the degree of brittle-ductile transformation of rock mass and the total damage variable, and form the inversion calculation rules for the damage evolution state of rock mass.

[0127] Furthermore, the specific steps for establishing the correlation system between physical property parameters and damage evolution are as follows:

[0128] First, based on the standardized test data obtained from S1 and the damage field data output from S4, quantitative correlations were established between the triaxial thermal conductivity, triaxial longitudinal wave velocity, and triaxial transverse wave velocity of dry hot rock and the degree of brittle-ductile conversion of the rock mass, as well as the total damage variable.

[0129] Then, based on the quantitative correlation, a calculation rule was established to invert the degree of brittle-ductile transformation of rock mass and the total damage variable through field measured physical property parameters;

[0130] Ultimately, a two-way correlation system between physical property parameters and damage evolution is formed, which can be both forward and inversely modeled. This two-way correlation system is used as the calculation basis for inverting the damage state of rock mass in the field.

[0131] Furthermore, the establishment of a two-way correlation system also includes the following steps:

[0132] Based on the spatiotemporal distribution data of acoustic emission collected by S1, a quantitative correlation was established between the degree of brittle-ductile transition of rock mass and the characteristic parameters of acoustic emission of rock mass.

[0133] By combining on-site microseismic monitoring data of the target reservoir with triaxial physical property parameters obtained from well logging, a dual-parameter collaborative inversion rule for the degree of brittle-ductile transformation of the rock mass and the total damage variable is constructed.

[0134] The dual-parameter collaborative inversion rule enables mutual verification between field monitoring data and well logging data, and collaboratively updates the characterization results of rock mass damage evolution.

[0135] Specifically, this step builds a bridge between the theoretical model and the engineering site for the entire technical system. The purpose of this step is to establish a quantitative correlation between rock mass physical parameters, brittle-ductile state, and damage variables, so as to realize the inversion of rock mass damage evolution state through field measured data and complete the engineering implementation of the theoretical model.

[0136] The specific implementation method is as follows: Based on the standardized test data obtained by S1 and the damage field data output by S4, a univariate control method is used to establish quantitative correlations between the three-dimensional thermal conductivity, three-dimensional P-wave velocity, three-dimensional S-wave velocity, and brittle-ductility conversion degree and total damage variable of hot dry rock. For the three principal stress directions, test data of rock thermal conductivity and P-wave and S-wave velocities are obtained under different values ​​of brittle-ductility conversion degree and total damage variable. Using a general exponential model for rock property attenuation, explicit correlation equations between the physical property parameters in each direction and the brittle-ductility conversion degree and total damage variable are established through nonlinear fitting. The general form is as follows:

[0137]

[0138] In the formula: i = 1, 2, 3, which correspond to the three principal stress directions respectively; , respectively, represent the thermal conductivity of the rock mass and the longitudinal wave velocity in the i-th direction; , respectively, represent the initial thermal conductivity and initial longitudinal wave velocity of the rock mass in the i-th direction under undamaged conditions, obtained from the experimental data of S1; a i b i c i d i These are the fitting coefficients for the corresponding directions, dimensionless, obtained from fitting experimental data.

[0139] Based on the above quantitative correlation, a two-way correlation system between physical property parameters and damage evolution is finally formed, which can be both forward and inverse: in the forward direction, the spatial distribution of physical property parameters of the rock mass can be predicted by the brittle-ductility conversion degree and total damage variable calculated by the numerical model; in the inverse direction, the distribution of brittle-ductility conversion degree and total damage variable of the rock mass can be inverted by the physical property parameters measured in the field, and the damage evolution state of the rock mass can be obtained.

[0140] During the establishment of the two-way correlation system, a quantitative correlation between the brittle-ductility conversion degree and the acoustic emission characteristic parameters of the rock mass was established based on the spatiotemporal distribution data of acoustic emission acquired by S1. Combining the on-site microseismic monitoring data of the target reservoir and the three-dimensional physical property parameter data obtained by well logging, a dual-parameter collaborative inversion rule for the brittle-ductility conversion degree and the total damage variable was constructed. The initial distribution of the total damage variable in the whole domain was obtained by inversion of well logging physical property data, and the spatial distribution of the brittle-ductility conversion degree was obtained by inversion of microseismic data. The two were cross-validated using the least squares method to collaboratively update the characterization results of the rock mass damage evolution state.

[0141] The established correlation system in this step enables the linkage between the theoretical numerical model and the measured data from the field engineering. The output inversion calculation rules will be directly applied to the subsequent field model verification and engineering characterization in S6.

[0142] S6. Verification and Characterization: Through indoor experiments and field monitoring data, the numerical model parameters are calibrated and the characterization accuracy is verified to complete the quantitative characterization of the damage evolution of the dry hot rock pressure fracture network.

[0143] Furthermore, the specific steps for verification and characterization are as follows:

[0144] First, the calculation results of the damage evolution numerical model built in S4 are compared with the experimental data through the indoor true triaxial hydraulic fracturing physical simulation test corresponding to S1, and the core parameters of the model are calibrated.

[0145] Then, the accuracy of the model damage characterization results was verified using microseismic monitoring data and well logging data from in-situ fracturing of the target hot dry rock reservoir.

[0146] Finally, the calibrated numerical model of damage evolution was applied to the quantitative characterization and dynamic analysis of fracture network damage evolution throughout the entire process of hydraulic fracturing in hot dry rock.

[0147] Specifically, this step involves completing indoor calibration and field verification of the entire technical system to achieve engineering application. The purpose of this step is to complete the parameter calibration and accuracy verification of the numerical model, ensure the engineering applicability of the method, and ultimately achieve quantitative characterization of the damage evolution of the dry hot rock pressure fracture network.

[0148] The specific implementation method is as follows: First, indoor model calibration is carried out. The true triaxial hydraulic fracturing physical simulation test corresponding to S1 is used. The boundary conditions, operating parameters, and initial physical property parameters of the test are input into the numerical model built by S4. The fracturing pressure, fracture propagation morphology, and acoustic emission event distribution calculated by the numerical model are compared with the experimental measured data. The core parameters of the model, such as the temperature influence coefficient, stress coupling influence coefficient, pore water pressure influence coefficient, and damage influence coefficient, are calibrated by trial and error to complete the indoor calibration of the model. Then, field model verification is carried out. The fracturing test well of the target dry hot rock reservoir is selected. The geological parameters of the reservoir, in-situ stress, in-situ temperature, and field fracturing construction parameters are input into the calibrated numerical model. The accuracy of the damage distribution and fracture network morphology inverted by the model is verified by the microseismic monitoring data, array acoustic logging data, and temperature logging data collected during the field fracturing process.

[0149] Finally, the calibrated and verified numerical model was applied to quantitatively characterize the fracture network damage evolution throughout the entire process of hydraulic fracturing in hot dry rock. This enables simulation of fracture network expansion before fracturing, real-time inversion of damage status during fracturing, and quantitative evaluation of fracturing effects after fracturing, providing theoretical support for the dynamic optimization of fracturing parameters in hot dry rock. This step completed the entire closed-loop process of model calibration from indoor experiments to field engineering verification, ensuring the engineering applicability of the characterization method and ultimately achieving accurate quantitative characterization of fracture network damage evolution in hot dry rock.

[0150] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model, characterized in that, Includes the following steps: S1. Data Acquisition: Conduct a true triaxial hydraulic fracturing orthogonal test covering the target hot dry rock reservoir conditions, and simultaneously collect relevant rock mass test data throughout the entire test process; S2. Construction of transformation criteria: Based on the experimental data obtained in S1, the influence of temperature, triaxial principal stress and pore water pressure on the characteristic stress of brittle-ductile transformation of dry hot rocks is analyzed, the boundary of the state interval of brittle-ductile transformation is determined, and the dynamic transformation threshold criterion rule for judging the degree of brittle-ductile transformation of rock mass is established. S3. Coupled Constitutive Modeling: Based on the dynamic transformation threshold criterion rule for brittleness and ductility established in S2, the components of the total damage variable of the rock mass are decomposed, a two-way linkage rule between the degree of brittleness and ductility transformation of the rock mass and the damage evolution is established, and a continuous damage mechanics framework is embedded to form a two-way coupled constitutive relationship of the rock mass. S4. Damage Model Construction: Based on the rock mass constitutive relationship established in S3, a numerical calculation framework with full coupling of four fields—temperature field, seepage field, stress field, and damage field—is constructed. A fracture propagation criterion matching the degree of brittle-ductile transformation of the rock mass is established, and the numerical model of damage evolution is completed, outputting damage field data. S5. Establishment of the correlation system: Based on the standardized test data obtained in S1 and the damage field data output in S4, establish the quantitative correlation between the three-dimensional physical property parameters of hot dry rock and the degree of brittle-ductile transformation of rock mass and the total damage variable, and form the inversion calculation rules for the damage evolution state of rock mass. S6. Verification and Characterization: Through indoor experiments and field monitoring data, the numerical model parameters are calibrated and the characterization accuracy is verified to complete the quantitative characterization of the damage evolution of the dry hot rock pressure fracture network.

2. The method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model according to claim 1, characterized in that, In step S1, the specific steps for collecting and processing the rock mass-related test data are as follows: First, orthogonal test conditions were set to cover the entire operating range of the target hot dry rock reservoir, including temperature, triaxial principal stress, and fracturing fluid injection rate. Then, a true triaxial hydraulic fracturing test was carried out, and rock mass stress-strain data, fracturing initiation and propagation pressure data, acoustic emission spatiotemporal distribution data, fracture three-dimensional morphology data, and cross-sectional fractal characteristic data were collected simultaneously throughout the entire test process. Finally, all collected data were standardized and outlier removed to obtain standardized experimental data, providing a standardized experimental data source for subsequent steps.

3. The method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model according to claim 1, characterized in that, In step S2, the specific steps for constructing the conversion criterion are as follows: First, based on the standardized test data obtained from S1, the independent and coupled effects of temperature, triaxial principal stress, and pore water pressure on the characteristic stress of brittle-ductile transition in hot dry rocks were quantitatively analyzed using the response surface methodology. Then, based on the variation range of characteristic stress in the brittle-ductile transition, the interval boundaries of the completely brittle state, the brittle-ductile transition state, and the completely ductile state of hot dry rock are determined; Finally, a dynamic threshold criterion rule for brittle-ductile transition that can be dynamically updated during the fracturing process and continuously determines the degree of brittle-ductile transition in the rock mass is established. This dynamic threshold criterion rule for brittle-ductile transition is used as the input basis for the bidirectional linkage rule in the subsequent step S3.

4. The method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model according to claim 1, characterized in that, In step S3, the specific steps for constructing the bidirectional coupled rock mass constitutive relationship are as follows: First, based on the dynamic conversion threshold criterion rule for brittleness and ductility established by S2, the total damage variable of the rock mass is decomposed into the brittle damage component generated by fracture tension and the ductile damage component generated by plastic deformation. Then, a real-time control rule is established that the higher the degree of brittle-ductile transition of the rock mass, the greater the weight of the ductile damage component and the smaller the weight of the brittle damage component. Then, establish a reverse correction rule that the larger the total damage variable of the rock mass, the lower the characteristic stress of brittle-ductile transition; Finally, the real-time control rules and the reverse correction rules are embedded into the continuous damage mechanics framework to form a rock mass constitutive relationship that is bidirectionally coupled with brittle-ductile transformation and damage evolution. The rock mass constitutive relationship is used as the core constitutive input for building the numerical model in the subsequent step S4.

5. The method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model according to claim 1, characterized in that, In step S4, the specific steps for building the numerical model of damage evolution are as follows: First, based on the constitutive relationship of the rock mass established by S3, the thermal conduction control equation of the temperature field, the seepage control equation of the seepage field, the mechanical equilibrium control equation of the stress field, and the evolution control equation of the damage field are simultaneously incorporated to construct a numerical calculation framework that fully couples the four fields of temperature field, seepage field, stress field, and damage field. Then, a tension-shear composite fracture propagation criterion matching the real-time brittle-ductile transformation degree of the rock mass was established; Finally, a damage evolution numerical model that can simultaneously characterize the dynamic propagation of three-dimensional fractures and the global damage evolution of hot dry rocks is established. The damage evolution numerical model outputs damage field data, which is used as input data for the subsequent step S5 to establish the correlation system.

6. The method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model according to claim 1, characterized in that, In step S4, the construction of the four-field fully coupled numerical computation framework also includes the following steps: A dynamic permeability evolution rule was simultaneously established, coupled with the real-time brittle-ductile transformation degree of the rock mass, total damage variable, and triaxial principal stress state. The pore water pressure distribution in the seepage field is corrected in real time by the aforementioned dynamic permeability evolution rule; The corrected pore water pressure is input in real time into the dynamic brittle-ductility conversion threshold criterion rule established by S2, and the degree of brittle-ductility conversion of rock mass and total damage variables are updated synchronously. A fully closed-loop linkage is formed between the seepage field and the rock mass constitutive model, and the updated rock mass brittle-ductile transformation degree and total damage variable are fed back into the fully coupled numerical calculation framework of the four fields.

7. The method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model according to claim 1, characterized in that, In step S4, the solution process of the damage evolution numerical model adopts a dynamic partitioning and hierarchical iterative solution mechanism, and the specific steps are as follows: In each iterative calculation step, the distribution of the rock mass brittle-ductile transformation degree in the entire calculation domain is first updated based on the temperature field, triaxial principal stress field, and pore water pressure field of the current step. Then, based on the degree of brittle-ductile transformation of the rock mass, the computational domain is dynamically divided into three independent computational zones: a completely brittle zone, a brittle-ductile transformation zone, and a completely ductile zone. Then, the rock mass constitutive relation, tension-shear composite fracture propagation criterion and calculation parameters are established for each zone according to the corresponding S3; After simultaneously solving the four-field fully coupled control equations for each partition, the solution results are used as input conditions for updating the distribution of rock mass brittle-ductile transformation degree in the next iteration step.

8. The method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model according to claim 1, characterized in that, In step S5, the specific steps for establishing the correlation system between the physical property parameters and damage evolution are as follows: First, based on the standardized test data obtained from S1 and the damage field data output from S4, quantitative correlations were established between the triaxial thermal conductivity, triaxial longitudinal wave velocity, and triaxial transverse wave velocity of dry hot rock and the degree of brittle-ductile conversion of the rock mass, as well as the total damage variable. Then, based on the quantitative correlation, a calculation rule is established to invert the degree of brittle-ductile transformation of rock mass and the total damage variable through field measured physical property parameters; Ultimately, a two-way correlation system between physical property parameters and damage evolution is formed, which can be both forward and inversely modeled. This two-way correlation system is used as the calculation basis for inverting the damage state of rock mass in the field.

9. The method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model according to claim 1, characterized in that, In step S5, the process of establishing the bidirectional correlation system also includes the following steps: Based on the spatiotemporal distribution data of acoustic emission collected by S1, a quantitative correlation was established between the degree of brittle-ductile transition of rock mass and the characteristic parameters of acoustic emission of rock mass. By combining on-site microseismic monitoring data of the target reservoir with triaxial physical property parameters obtained from well logging, a dual-parameter collaborative inversion rule for the degree of brittle-ductile transformation of the rock mass and the total damage variable is constructed. The dual-parameter collaborative inversion rule enables mutual verification between field monitoring data and well logging data, and collaboratively updates the characterization results of rock mass damage evolution.

10. The method for characterizing the damage evolution of a hot dry rock pressure fracture network based on a brittle-ductile transformation constitutive model according to claim 1, characterized in that, In step S6, the specific steps of verification and characterization are as follows: First, the calculation results of the damage evolution numerical model built in S4 are compared with the experimental data through the indoor true triaxial hydraulic fracturing physical simulation test corresponding to S1, and the core parameters of the model are calibrated. Then, the accuracy of the model damage characterization results was verified using microseismic monitoring data and well logging data from in-situ fracturing of the target hot dry rock reservoir. Finally, the calibrated numerical model of damage evolution was applied to the quantitative characterization and dynamic analysis of fracture network damage evolution throughout the entire process of hydraulic fracturing in hot dry rock.

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