Deep fracture-bedrock interbedded type hot dry rock heat energy exploitation method

Abstract

The present application relates to the technical field of deep geothermal energy development, and particularly relates to a deep fracture-bedrock interbedded type hot dry rock heat energy mining method, comprising the following steps: constructing a discrete fracture network model, considering porous medium heat transfer, Darcy's law and solid mechanics, that is, heat-flow-solid physical field, revising the discrete fracture network model; for the revised discrete fracture network model, three different injection-production modes of upper injection and lower production, lower injection and upper production and same layer injection and production are simulated respectively; according to the relationship between the water production temperature of the production well, the injection speed of the heat-carrying medium, the injection temperature and the production pressure difference, the optimal injection-production mode and injection-production parameters of the fracture-bedrock interbedded development heat reservoir are selected. The present application can describe the hot dry rock reservoir in detail by establishing a discrete fracture network model, which is closer to the real scene of the hot dry rock reservoir, so that the simulation result is more real and reliable.

Description

Technical Field

[0001] This invention relates to the field of deep geothermal energy development technology, specifically to a method for extracting thermal energy from dry hot rock in a deep fracture-bedrock interbedded type. Background Technology

[0002] With the increasing severity of the greenhouse effect, energy conservation and emission reduction have gradually become a focus of attention in response to global warming. Geothermal energy, as a clean and environmentally friendly renewable energy source, has gradually become a key area of ​​research and development for countries around the world. Hot dry rock (HDR) is an important geothermal resource, generally referring to high-temperature rock masses buried 3-10 km underground, containing little or no fluid and with temperatures above 180°C, storing approximately 90% or more of the total geothermal energy. According to literature surveys, the heat storage capacity of hot dry rock in the United States exceeds 1.4 × 10⁻⁶. 25 J, its thermal reserves in China are approximately 2.5 × 10⁻⁶. 25 J.

[0003] Enhanced geothermal systems (EGS) extract and utilize geothermal energy by injecting a low-temperature heat-carrying medium into deep, high-temperature, dry hot rock reservoirs. The medium flows between injection and production wells. The low-temperature medium exchanges heat with the deep, high-temperature rock mass. Due to the high temperature of the dry hot rock reservoir, heat is transferred from the hot rock mass to the low-temperature medium, causing the injected medium to heat up through this exchange. Deep geothermal energy extraction involves the complex spatiotemporal evolution of hydrodynamic, stress, and temperature fields within fractured rock masses—a thermal-fluid-solid coupling problem. This method comprehensively considers the coupling effects between these fields, more closely resembling the actual geological conditions of dry hot rock reservoirs. Due to differences in geological movement, in-situ stress conditions, petrology, and rock mass fracturing capabilities, geothermal reservoirs exhibit strong heterogeneity in their storage and permeability at the reservoir site scale. Literature review indicates that existing deep geothermal energy development methods and construction parameter optimizations are all based on single fracture geological models or single bedrock geological models. Fracture characteristic parameters are homogeneously equivalent, making it difficult to reflect the impact of fracture system density, occurrence, and spatial connectivity on the heat exchange efficiency of the heat-carrying medium. No publicly reported feasibility studies have been published on deep geothermal energy development methods based on interbedded geological models of fracture-bedrock symbiotic systems. However, due to the complexity of natural and artificial fractures in the rock mass, in the plane, after the injection of low-temperature heat-carrying medium, it will advance along the dominant direction of the fracture network; vertically, high-conductivity fracture layers and dense bedrock layers develop interbedded, and after the injection of heat-carrying medium, the low-temperature front exhibits finger-like breakthroughs, thus significantly affecting the heat recovery rate and operational life of the geothermal field. Research on extraction methods for different injection and extraction approaches and the vertical interbedded development of fractures in hot dry rock reservoirs is lacking.

[0004] The interaction between injection wells and production wells in hot dry rock reservoirs affects the thermal extraction efficiency of the heat-carrying medium. Due to different injection and production methods, the reach of the heat-carrying medium in hot dry rock reservoirs varies, consequently affecting its thermal extraction efficiency. Currently, research on well types is extensive, but studies on injection and production methods and construction parameters for fracture-bedrock interbedded geothermal reservoirs are still lacking. Therefore, further research is needed on the multi-physics field of geothermal reservoirs and the development effects of fracture-bedrock interbedded structures under different injection and production methods to more efficiently develop and utilize my country's abundant geothermal resources.

[0005] Chinese patent CN112984849B, entitled "Method for Geothermal Development of Cambrian Karst Thermal Reservoirs and Fractured Thermal Reservoirs in Metamorphic Strata," discloses a method for developing geothermal reservoirs in Cambrian karst thermal reservoirs and fractured thermal reservoirs in metamorphic strata. It primarily addresses technical issues encountered during current geothermal development, such as low extraction temperatures from shallow karst thermal reservoirs, impacting residential water supply, and the inability to achieve 100% reinjection.

[0006] Chinese patent CN115573692A, entitled "System and Method for Constructing Artificial Thermal Reservoirs in Hot Dry Rocks," discloses a system and method for constructing artificial thermal reservoirs in hot dry rocks. It constructs a vertical fracture system, which makes formation fracturing easier and allows for a wider fracturing range.

[0007] Chinese patent CN114429011A, entitled "Method, Apparatus, and Medium for Calculating the Total Heat Transfer Surface of Fracturing in Hot Dry Rock Reservoirs," discloses a method, apparatus, and medium for calculating the total heat transfer surface of fracturing in hot dry rock reservoirs. This invention establishes the relationship between the power generation of the surface system and the parameters of artificial fractures in the underground reservoir. However, this patent does not address geothermal resource development technologies that consider the vertical heterogeneity of the fracture layer-bedrock layer and the coupling of multiple physical fields.

[0008] None of the aforementioned patents considered the heterogeneity of the reservoir space and failed to provide a true and effective description of the geometry of the fracture system. Summary of the Invention

[0009] The present invention aims to provide a method for the thermal energy extraction of dry hot rock considering the interaction between multiple physical fields, the longitudinal superposition and development of fracture layers, and different injection and extraction methods, in order to solve the problem of the lack of current methods for the thermal energy extraction of deep fracture-bedrock interbedded dry hot rock reservoirs.

[0010] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A method for extracting thermal energy from deep fractured-bedrock interbedded dry hot rock, comprising the following steps:

[0011] Step 1: Based on the results of borehole core temperature monitoring, geological outcrop investigation, borehole core fracture description, and geophysical fracture prediction, determine the reservoir temperature and the distribution status and form of fractures in the deep high-temperature dry hot rock reservoir.

[0012] Step 2: Based on the reservoir temperature and the distribution state and form of fractures in the deep high-temperature dry hot rock reservoir obtained in Step 1, a discrete fracture network model of the deep high-temperature dry hot rock reservoir is established using existing software in the field, in which fracture layers and bedrock layers are vertically superimposed in multiple layers.

[0013] Step 3: Considering heat transfer in porous media, Darcy's law, and solid mechanics (i.e., the thermo-fluid-solid physical field), the discrete crack network model is revised as follows:

[0014] The flow of the heat-carrying working fluid in the above discrete crack network model is described by the law of conservation of mass (2-1):

[0015]

[0016] in The density of the heat-carrying working fluid, The porosity of deep, high-temperature, dry, and hot rocks. To simulate time, It serves as a mass source in deep, high-temperature, dry, and hot rocks. The flow rate of the heat-carrying medium in deep, high-temperature, dry hot rock;

[0017] The motion of the heat-carrying working fluid is described by Darcy's law (3-1):

[0018]

[0019] in The permeability of deep, high-temperature, dry, and hot rock. The viscosity of the heat-carrying working fluid. For pressure gradient, This represents the depth gradient of deep, high-temperature, dry, hot rock reservoirs.

[0020] The heat transfer process in the porous medium in the discrete crack network model is described by the energy conservation equation (4-1):

[0021]

[0022] in, ; and These are the effective heat capacity and effective thermal conductivity obtained by volume averaging, respectively. This refers to the reservoir temperature of deep, high-temperature, dry, and hot rocks. The density of deep, high-temperature, dry, hot rock. For specific heat capacity, For thermal conductivity, the subscript " "and" "Represents the solid phase and the liquid phase respectively; The constant-pressure specific heat capacity of deep, high-temperature, dry hot rock. The specific heat capacity of the heat-carrying medium within deep, high-temperature, dry hot rock. This represents the temperature gradient of deep, high-temperature, dry, hot rocks;

[0023] The mechanical deformation of deep, high-temperature, dry hot rocks is based on the thermo-pore-elastic principle. By combining the effects of thermal expansion and pore pressurization, the governing equation (5-1) for matrix deformation is derived as follows:

[0024]

[0025] in It is the effective stress. Normal compressive stress, It is a response. It is Young's modulus. It is Poisson's ratio. It is the Biot coefficient. It is the pressure of the heat-carrying working fluid. It is the Kronecker function. It is the coefficient of thermal expansion. It is the temperature increment. , and These are the temperature of the deep high-temperature dry hot rock and the initial reference temperature of the injected heat-carrying working fluid, respectively.

[0026] The temperature field equation (6-1) for deep, high-temperature, dry hot rocks is:

[0027]

[0028] In the formula, The density of deep, high-temperature, dry, hot rock; Cs is the thermal conductivity coefficient of deep high-temperature dry hot rock; Cs is the specific heat capacity of deep high-temperature dry hot rock. For the Laplacian operator, is the divergence of the gradient; T s t is the surface temperature of deep, high-temperature, dry hot rock; W is the heat source.

[0029] Equation for the temperature field of fracture water (7-1):

[0030]

[0031] In the formula, The crack opening; The density of the heat-carrying working fluid; Specific heat capacity of the heat-carrying working fluid; The temperature of the heat-carrying working fluid; The flow rate of the heat-carrying working fluid; The thermal conductivity coefficient of the heat-carrying working fluid; To differentiate along the crack tangentially; The velocity of the heat-carrying working fluid within the crack; The temperature of the heat-carrying medium within the crack; This refers to the heat absorbed by the heat-carrying medium on the surface of the crack from the deep, high-temperature, dry, hot rock.

[0032] The heat exchange between the heat-carrying medium within the fracture and the deep high-temperature dry hot rock follows Newton's heat transfer formula. The heat exchange at the boundary between the heat-carrying medium and the deep high-temperature dry hot rock is calculated using the convective heat transfer coefficient. That is, it is assumed that the average temperature of the heat-carrying medium within the fracture is different from the surface temperature of the deep high-temperature dry hot rock. The heat transferred per unit area from the deep high-temperature dry hot rock to the heat-carrying medium within the fracture is (8-1):

[0033]

[0034] In the formula, The heat transfer coefficient, It refers to the surface temperature of deep, high-temperature, dry, and hot rock.

[0035] Step 4: Simulate the discrete crack network model revised in Step 3 using three different injection and extraction methods: top injection and bottom extraction, bottom injection and top extraction, and same-layer injection and extraction.

[0036] Step 5: For the revised discrete crack network model, the three injection and extraction methods proposed in Step 4 are applied. For each injection and extraction method, the injection rate q of the heat-carrying working fluid is changed. inj Injection temperature T inj The production pressure difference ΔP was obtained using numerical simulation methods, showing the production well water temperature as a function of q during the extraction time. inj T inj Based on the results of the change in ΔP, with the extraction time of deep high-temperature dry hot rock thermal energy as the abscissa and the production well water temperature as the ordinate, a temperature change curve of the heat-carrying medium at the production well outlet is plotted, thus obtaining the temperature T of the heat-carrying medium at the production well outlet. out The changing pattern.

[0037] Step Six: Set the injection speed q inj Injection temperature T inj With different values ​​of the production pressure difference ΔP and construction parameters, N sets of test schemes for construction parameter combinations are established using orthogonal experimental methods.

[0038] According to the method described in step five, N production well water temperature change curves are obtained. The range analysis method is used to analyze the N production well water temperature change curves, and the optimal production well water temperature change curve is selected. Based on the process parameters represented by the optimal production well water temperature change curve, thermal energy is extracted from the target dry hot rock reservoir.

[0039] Preferably, in the discrete fracture network model established in step two, the fractures are treated as linear units with no thickness, and the deep high-temperature dry hot rock is discretized using solid units. The fracture length distribution of the fracture network usually adopts a power-law distribution model, and the relationship between the number of fractures and their length is expressed as (1-1):

[0040]

[0041] In the formula, The size of the field; The length of the crack; For a square reservoir with side length L, the fracture length l is given by... The number of cracks inside; It is the fractal dimension; This is the crack length index; It is a constant related to the crack density; and These represent the maximum and minimum fracture lengths within a square reservoir, respectively.

[0042] Compared with existing technologies, the present invention has the following advantages:

[0043] 1. This invention fully discusses the advantages and disadvantages of different injection and production methods for developing the thermal energy of deep dry hot rock reservoirs, and optimizes reasonable injection and production parameters in combination with the water production temperature of production wells, providing a new approach for the subsequent development of thermal energy in deep dry hot rock reservoirs.

[0044] 2. This invention considers the interaction between three different physical fields: heat, fluid, and solid, and solves the pain points and difficulties in the development of deep dry hot rock reservoirs, realizing the research on the optimization of dry hot rock injection and production methods.

[0045] 3. This invention provides a realistic and detailed description of hot dry rock reservoirs by establishing a discrete fracture network model, which is closer to the real-world hot dry rock reservoir scenario and makes the simulation results more realistic and reliable. Attached Figure Description

[0046] Figure 1 This is a flowchart of the method of the present invention.

[0047] Figure 2 This is a geological model of a fracture-bedrock interbedded dry hot rock with an upper injection and lower mining method.

[0048] Figure 3A geological model of a fracture-bedrock interbedded dry hot rock with a bottom-up mining method.

[0049] Figure 4 This is a geological model of a fracture-bedrock interbedded dry hot rock with the same injection and extraction method.

[0050] Figure 5 The diagram shows the relationship between the water production temperature of the production wells after 20 years of mining for the three injection and production methods in the example.

[0051] Figure 6 This is a graph showing the relationship between the water production temperature of the production well after 20 years of extraction using the same injection-production method under different mass flow rates of the heat-carrying working fluid in the example.

[0052] Figure 7 This is a graph showing the relationship between the water production temperature of production wells after 20 years of extraction using the same injection and production method at different injection temperatures of the heat-carrying working fluid in the example.

[0053] Figure 8 This is a graph showing the relationship between the water production temperature of production wells after 20 years of extraction using the same injection-production method under different production pressure differentials in the example. Detailed Implementation

[0054] The present invention will now be described in detail with reference to the accompanying drawings.

[0055] See Figures 1-4 A method for extracting thermal energy from deep fractured bedrock interbedded dry hot rock includes the following steps:

[0056] Step 1: Based on borehole core temperature monitoring, geological outcrop investigation, borehole core fracture description, and geophysical fracture prediction results, determine the reservoir temperature and the distribution state and form of fractures in the deep high-temperature dry hot rock reservoir. This step, which uses borehole core temperature monitoring, geological outcrop investigation, borehole core fracture description, and geophysical fracture prediction results to determine the reservoir temperature and the distribution state and form of fractures in the deep high-temperature dry hot rock reservoir, is existing technology. In addition, the deep high-temperature dry hot rock in this invention refers to deep fracture-bedrock interbedded dry hot rock.

[0057] Step Two: Based on the reservoir temperature and fracture distribution patterns of the deep high-temperature dry hot rock reservoir obtained in Step One, a discrete fracture network model of the deep high-temperature dry hot rock reservoir is established using existing software in the field. This model features multiple vertically stacked fracture layers and bedrock layers. The fractures will be characterized by their location, orientation, and aperture, i.e., represented using a discrete fracture method. A discrete fracture network model of the deep high-temperature dry hot rock reservoir is then established using Matlab software.

[0058] Step 3: Considering heat transfer in porous media, Darcy's law, and solid mechanics, i.e., the heat-fluid-solid physical field, the discrete fracture network model is revised as follows: The discrete fracture network model uses water as the heat-carrying medium to exchange heat with the rock matrix established by the model, thereby extracting heat.

[0059] The flow of the heat-carrying working fluid in the above discrete crack network model is described by the law of conservation of mass (2-1):

[0060]

[0061] in The density of the heat-carrying working fluid, The porosity of deep, high-temperature, dry, and hot rocks. To simulate time, It serves as a mass source in deep, high-temperature, dry, and hot rocks. The flow rate of the heat-carrying medium in deep, high-temperature, dry hot rock;

[0062] The motion of the heat-carrying working fluid is described by Darcy's law (3-1):

[0063]

[0064] in The permeability of deep, high-temperature, dry, and hot rock. The viscosity of the heat-carrying working fluid. For pressure gradient, This represents the depth gradient of deep, high-temperature, dry, hot rock reservoirs.

[0065] The heat transfer process in the porous medium in the discrete crack network model is described by the energy conservation equation (4-1):

[0066]

[0067] in, ; and These are the effective heat capacity and effective thermal conductivity obtained by volume averaging, respectively. This refers to the reservoir temperature of deep, high-temperature, dry, and hot rocks. The density of deep, high-temperature, dry, hot rock. For specific heat capacity, For thermal conductivity, the subscript " "and" "Represents the solid phase and the liquid phase respectively; The constant-pressure specific heat capacity of deep, high-temperature, dry hot rock. The specific heat capacity of the heat-carrying medium within deep, high-temperature, dry hot rock. This represents the temperature gradient of deep, high-temperature, dry, hot rocks.

[0068] The mechanical deformation of deep, high-temperature, dry hot rocks is based on the thermo-pore-elastic principle. By combining the effects of thermal expansion and pore pressurization, the governing equation (5-1) for matrix deformation is derived as follows:

[0069]

[0070] in It is the effective stress. Normal compressive stress, It is a response. It is Young's modulus. It is Poisson's ratio. It is the Biot coefficient. It is the pressure of the heat-carrying working fluid. It is the Kronecker function. It is the coefficient of thermal expansion. It is the temperature increment. , and These are the temperature of the deep high-temperature dry hot rock and the initial reference temperature of the injected heat-carrying working fluid, respectively.

[0071] The temperature field equation (6-1) for deep, high-temperature, dry hot rocks is:

[0072]

[0073] In the formula, The density of deep, high-temperature, dry, hot rock; Cs is the thermal conductivity coefficient of deep high-temperature dry hot rock; Cs is the specific heat capacity of deep high-temperature dry hot rock. For the Laplacian operator, is the divergence of the gradient; T s t is the surface temperature of deep, high-temperature, dry hot rock; W is the heat source.

[0074] Equation for the temperature field of fracture water (7-1):

[0075]

[0076] In the formula, The crack opening; The density of the heat-carrying working fluid; Specific heat capacity of the heat-carrying working fluid; The temperature of the heat-carrying working fluid; The flow rate of the heat-carrying working fluid; The thermal conductivity coefficient of the heat-carrying working fluid; To differentiate along the crack tangentially; The velocity of the heat-carrying working fluid within the crack; The temperature of the heat-carrying medium within the crack; The heat absorbed by the heat-carrying medium on the surface of the crack from the deep, high-temperature, dry rock; the second item " "This indicates the convection effect of the heat-carrying medium within the crack on the temperature field."

[0077] The heat exchange between the heat-carrying medium within the fracture and the deep high-temperature dry hot rock follows Newton's heat transfer formula. The heat exchange at the boundary between the heat-carrying medium and the deep high-temperature dry hot rock is calculated using the convective heat transfer coefficient. That is, it is assumed that the average temperature of the heat-carrying medium within the fracture is different from the surface temperature of the deep high-temperature dry hot rock. The heat transferred per unit area from the deep high-temperature dry hot rock to the heat-carrying medium within the fracture is (8-1):

[0078]

[0079] In the formula, The heat transfer coefficient, It refers to the surface temperature of deep, high-temperature, dry, and hot rock.

[0080] Step 4: Due to spatial heterogeneity and the high conductivity effect of fractures, during heat extraction in deep high-temperature dry hot rock reservoirs with interbedded fracture and bedrock layers, the non-uniform advancement of the low-temperature heat-carrying medium front in both the horizontal and vertical directions causes premature breakthroughs in some production well sections, thus affecting heat extraction efficiency. To address this engineering challenge, based on the discrete fracture network model constructed in Step 3, simulations were conducted using three different injection and extraction methods: top-injection-bottom-production, bottom-injection-top-production, and same-layer injection and extraction. Taking a three-layer fracture superposition as an example, the fracture layer was divided into upper, middle, and lower layers, and the three different injection and extraction methods were: top-injection-bottom-production, bottom-injection-top-production, and same-layer injection and extraction. In the above-injection and below-production model, the left end of the upper fracture layer is the injection end (i.e., the injection well), and the right end of the lower fracture layer is the outlet end (i.e., the production well). In the below-injection and above-production model, the right end of the upper fracture layer is the outlet end (i.e., the production well), and the left end of the lower fracture layer is the injection end (i.e., the injection well). In the same-layer injection-production model, the left end of the dry hot rock reservoir (i.e., the left end of the upper, middle, and lower fracture layers) and the left end of the reservoir matrix rock are set as the injection end (i.e., the injection well), and the right end is set as the outlet end (i.e., the production well).

[0081] Step 5: For the revised discrete crack network model, the three injection and extraction methods proposed in Step 4 are applied. For each injection and extraction method, the injection rate q of the heat-carrying working fluid is changed. inj Injection temperature T inj The production pressure difference ΔP was obtained using numerical simulation methods, showing the production well water temperature as a function of q during the extraction time. inj T inj Based on the results of the change in ΔP, with the extraction time of deep high-temperature dry hot rock thermal energy as the abscissa and the production well water temperature as the ordinate, a temperature change curve of the heat-carrying medium at the production well outlet is plotted, thus obtaining the temperature T of the heat-carrying medium at the production well outlet. out The changing pattern; that is, for a certain injection method, the injection rate q inj Injection temperature T injThe production pressure difference ΔP is used as the input to the revised discrete fracture network model, and the output of the revised discrete fracture network model is the water production temperature of the production well under this injection-production method. This is achieved by continuously changing q. inj T inj And ΔP, to obtain the corresponding production well water temperature, and then draw the temperature change curve of the heat-carrying working medium at the outlet of the production well based on the obtained production well water temperature; the same method is used for the other two injection and production methods.

[0082] Based on the initial temperature, reservoir thickness, number of stacked layers, fracture density, and injection-production well spacing of the target hot dry rock reservoir, the construction parameters are optimized mainly for both injection and production wells. For injection wells, the parameter to be optimized is the injection rate q. inj Injection temperature T inj For production wells, the main optimization is the production pressure differential ΔP. Finally, the production water temperature T of the production well is considered. out As an evaluation indicator of the effectiveness of geothermal development.

[0083] Step Six: Set the injection speed q inj Injection temperature T inj With different values ​​of the production pressure difference ΔP and construction parameters, N sets of test schemes for construction parameter combinations are established using orthogonal experimental methods.

[0084] According to the method described in step five, N production well water temperature change curves are obtained. The range analysis method is used to analyze the N production well water temperature change curves, and the optimal production well water temperature change curve is selected. Based on the process parameters represented by the optimal production well water temperature change curve, thermal energy is extracted from the target dry hot rock reservoir.

[0085] Specifically, in the discrete fracture network model established in step two, the fractures are treated as linear units with no thickness, and the deep high-temperature dry hot rock is discretized using solid units. The fracture length distribution of the fracture network usually adopts a power-law distribution model, and the relationship between the number of fractures and their length is expressed as (1-1):

[0086]

[0087] In the formula, The size of the field; The length of the crack; For a square reservoir with side length L, the fracture length l is given by... The number of cracks inside; It is the fractal dimension; This is the crack length index; It is a constant related to the crack density; and These represent the maximum and minimum fracture lengths within a square reservoir, respectively.

[0088] Example 1:

[0089] Taking the granite buried hill geothermal field in Block H of the Bangor Basin in Chad, Central Africa, as the target, seismic interpretation, well logging analysis, and comprehensive geological studies indicate that the target reservoir is a granitic fractured buried hill dry hot rock reservoir, with a burial depth ranging from 2800m to 3340m. The fractured layers and bedrock layers are interbedded from the unconformity surface to the interior of the buried hill, representing a typical fracture-bedrock interbedded geothermal reservoir with extremely strong vertical heterogeneity. Drilling temperature measurements and core descriptions show that the bottom-hole temperature of the dry hot rock reservoir is 260℃~290℃, the average fracture length is 12 meters, and the fracture dip angle is 20°~80°. The dry hot rock reservoir in this geothermal field exhibits layered and superimposed fractures, with the fractured layers divided into upper, middle, and lower layers.

[0090] Based on the above analysis results, geothermal extraction was carried out on the granite buried hill geothermal field in Block H of the Bangor Basin in Chad, Central Africa. The specific extraction steps are as follows:

[0091] Step 1: Based on the relevant parameters of the aforementioned hot dry rock reservoir, a discrete fracture network model was established using numerical simulation. The model is 3000 m long and 1240 m wide. The average fracture length is 8 meters, and the fracture dip angle ranges from 20° to 80°, with an average of 65°. The fracture layer is divided into upper, middle, and lower layers. Detection indicates that the initial temperature of the hot dry rock reservoir is 270°C. Therefore, water with an initial temperature of 20°C was used as the low-temperature heat-carrying medium for thermal energy extraction from this hot dry rock reservoir.

[0092] Step 2: Consider the heat transfer in porous media, Darcy's law, and solid mechanics for the physical model of the longitudinally superimposed fracture layer and bedrock layer.

[0093] Step 3: Based on the discrete fracture network model described above, establish different injection and production models for three injection and production methods: top-injection-bottom-production, bottom-injection-top-production, and same-layer injection and production. All three methods predict the production well water temperature T under a 20-year thermal recovery period. out like Figure 5 The higher the temperature, the more efficient the heat exchange between the heat-carrying medium and the high-temperature rock mass, resulting in better heat extraction. Figure 5 It can be seen that after 20 years of operation, the water production temperature of the production well in the top-injection-bottom-production model is 293.45 K, the water production temperature of the production well in the bottom-injection-top-production model is 294.56 K, and the water production temperature of the production well in the same-layer injection-production model is 297.05 K. Therefore, this hot dry rock reservoir is suitable for the same-layer injection-production development method, that is, the injection well is located at one end of the hot dry rock reservoir, which is set as the left end, and the production well is located at the other end of the hot dry rock reservoir, which is set as the right end.

[0094] Step 4: Develop the hot dry rock reservoir based on the selected injection-production method in Step 3. Set the exploitation period of the hot dry rock reservoir to 20 years. Inject a heat-carrying medium at a temperature of 20℃ at mass flow rates of 1 kg / s, 5 kg / s, and 10 kg / s respectively. Using numerical simulation, obtain the temperature change curves of the heat-carrying medium at the production well outlet under different injection rates during the 20-year exploitation period, with an injection-production pressure difference of 5 MPa. Figure 6 With all other parameters remaining the same, a mass flow rate of 8 kg / s for the heat-carrying medium, and an injection-production pressure difference of 5 MPa, the injection temperatures of the heat-carrying medium were changed to 10℃, 15℃, and 20℃. Numerical simulation was used to obtain the temperature variation curves at the production well outlet under different injection temperatures of the heat-carrying medium. Figure 7 Under the same conditions as described above, with an injection temperature of 20℃ and a mass flow rate of 8 kg / s for the heat-carrying medium, thermal energy extraction was carried out on this dry hot rock reservoir using production pressure differentials of 4 MPa, 6 MPa, and 8 MPa. Numerical simulation was used to obtain the temperature variation curves of the heat-carrying medium at the production well outlet over 20 years under different pressure differentials. Figure 8 ).

[0095] Step 5: Based on the numerical simulation results above, the temperature of the heat-carrying medium at the production well outlet and the injection rate q of the heat-carrying medium are obtained. inj Injection temperature T inj The relationship curve between production pressure difference ΔP and the pressure difference. Figure 6 This indicates that if T is used out With a production target of >393.15K, the system's operational lifespan is 9 years, 7 years, and 5 years for heat-carrying medium mass flow rates of 1 kg / s, 5 kg / s, and 10 kg / s, respectively. The system with a mass flow rate of 1 kg / s has an operational lifespan 4 years longer than the system with a mass flow rate of 10 kg / s. Furthermore, when the heat-carrying medium mass flow rate is 1 kg / s, the permeable water temperature at any given time is higher than that of the other two types of permeable wells. Figure 7 It can be seen that when the heat-carrying medium is injected at temperatures of 10℃, 15℃, and 20℃, the system's operational lifespan is 5 years, 7 years, and 8 years, respectively. Therefore, when the heat-carrying medium is injected at 20℃, the system has the longest operational lifespan, and after 20 years of operation, the produced well's water temperature is the highest compared to the other two types. Figure 8 This indicates that the system's operating life is 8 years, 6 years, and 4 years at production pressure differentials of 4 MPa, 6 MPa, and 8 MPa, respectively. That is, when the production pressure differential of the production well is 4 MPa, the system's operating life is the longest at 8 years. Furthermore, the water temperature produced by the production well under this production pressure differential is higher than that under the production pressure differentials of 6 MPa and 8 MPa after 20 years of extraction.

[0096] The above results indicate that the thermal energy extraction effect of this fracture-bedrock interbedded dry hot rock reservoir is best when the injection rate of the heat-carrying medium is 1 kg / s, the injection temperature of the heat-carrying medium is 20℃, and the production pressure difference of the production well is 4 MPa. These parameters are the optimal injection and production parameters for this granite buried hill geothermal field.

[0097] The above embodiments are only used to illustrate the present invention. The implementation steps and related parameters may vary. Any equivalent transformations and improvements made on the basis of the technical solution of the present invention should not be excluded from the protection scope of the present invention.

Claims

1. A method for extracting thermal energy from dry hot rock of deep fracture-bedrock interbedded type, characterized in that: Includes the following steps: Step 1: Based on the results of borehole core temperature monitoring, geological outcrop investigation, borehole core fracture description and geophysical fracture prediction, determine the reservoir temperature and the distribution status and form of fractures in the deep high-temperature dry hot rock reservoir. Step 2: Based on the reservoir temperature and the distribution state and form of fractures in the deep high-temperature dry hot rock reservoir obtained in Step 1, a discrete fracture network model of the deep high-temperature dry hot rock reservoir is established using existing software in the field, in which fracture layers and bedrock layers are vertically superimposed in multiple layers. Step 3: Considering heat transfer in porous media, Darcy's law, and solid mechanics (i.e., the thermo-fluid-solid physical field), the discrete crack network model is revised as follows: The flow of the heat-carrying working fluid in the above discrete crack network model is described by the law of conservation of mass (2-1): in The density of the heat-carrying working fluid, The porosity of deep, high-temperature, dry, and hot rocks. To simulate time, It serves as a mass source in deep, high-temperature, dry, and hot rocks. The flow rate of the heat-carrying medium in deep, high-temperature, dry hot rock; The motion of the heat-carrying working fluid is described by Darcy's law (3-1): in The permeability of deep, high-temperature, dry, and hot rock. The viscosity of the heat-carrying working fluid. For pressure gradient, This represents the depth gradient of deep, high-temperature, dry, and hot rock reservoirs. The heat transfer process in the porous medium in the discrete crack network model is described by the energy conservation equation (4-1): in, ; and These are the effective heat capacity and effective thermal conductivity obtained by volume averaging, respectively. This refers to the reservoir temperature of deep, high-temperature, dry, and hot rocks. The density of deep, high-temperature, dry, hot rock. For specific heat capacity, The value is the thermal conductivity, indicated by the subscript "". "and" "Represents the solid phase and the liquid phase respectively; The constant-pressure specific heat capacity of deep, high-temperature, dry hot rock. The specific heat capacity of the heat-carrying medium within deep, high-temperature, dry hot rock. This represents the temperature gradient of deep, high-temperature, dry, hot rocks; The mechanical deformation of deep, high-temperature, dry hot rocks is based on the thermo-pore-elastic principle. By combining the effects of thermal expansion and pore pressurization, the governing equation (5-1) for matrix deformation is derived as follows: in It is the effective stress. Normal compressive stress, It is a response. It is Young's modulus. It is Poisson's ratio. It is the Biot coefficient. It is the pressure of the heat-carrying working fluid. It is the Kronecker function. It is the coefficient of thermal expansion. It is the temperature increment. , and These are the temperature of the deep high-temperature dry hot rock and the initial reference temperature of the injected heat-carrying working fluid, respectively. The temperature field equation (6-1) for deep, high-temperature, dry hot rocks is: In the formula, The density of deep, high-temperature, dry, hot rock; Cs is the thermal conductivity coefficient of deep high-temperature dry hot rock; Cs is the specific heat capacity of deep high-temperature dry hot rock. For the Laplacian operator, is the divergence of the gradient; T s The surface temperature of deep, high-temperature, dry hot rock; W is the heat source; Equation for the temperature field of fracture water (7-1): In the formula, The crack opening; The density of the heat-carrying working fluid; Specific heat capacity of the heat-carrying working fluid; The temperature of the heat-carrying working fluid; The flow rate of the heat-carrying working fluid; The thermal conductivity coefficient of the heat-carrying working fluid; To differentiate along the crack tangentially; The velocity of the heat-carrying working fluid within the crack; The temperature of the heat-carrying medium within the crack; The heat absorbed by the heat-carrying medium on the surface of the crack from the deep, high-temperature, dry hot rock; The heat exchange between the heat-carrying medium within the fracture and the deep high-temperature dry hot rock follows Newton's heat transfer formula. The heat exchange at the boundary between the heat-carrying medium and the deep high-temperature dry hot rock is calculated using the convective heat transfer coefficient. That is, it is assumed that the average temperature of the heat-carrying medium within the fracture is different from the surface temperature of the deep high-temperature dry hot rock. The heat transferred per unit area from the deep high-temperature dry hot rock to the heat-carrying medium within the fracture is (8-1): In the formula, The heat transfer coefficient, This refers to the surface temperature of deep, high-temperature, dry, and hot rock. Step 4: Simulate the discrete crack network model revised in Step 3 using three different injection and extraction methods: top injection and bottom extraction, bottom injection and top extraction, and same-layer injection and extraction. Step 5: For the revised discrete crack network model, the three injection and extraction methods proposed in Step 4 are applied. For each injection and extraction method, the injection rate q of the heat-carrying working fluid is changed. inj Injection temperature T inj The production pressure difference ΔP was obtained using numerical simulation methods, showing the production well water temperature as a function of q during the extraction time. inj T inj Based on the results of the change in ΔP, with the extraction time of deep high-temperature dry hot rock thermal energy as the abscissa and the production well water temperature as the ordinate, a temperature change curve of the heat-carrying medium at the production well outlet is plotted, thus obtaining the temperature T of the heat-carrying medium at the production well outlet. out The changing pattern; Step Six: Set the injection speed q inj Injection temperature T inj With different values ​​of the production pressure difference ΔP and construction parameters, N sets of test schemes for construction parameter combinations are established using orthogonal experimental methods. According to the method described in step five, N production well water temperature change curves are obtained. The range analysis method is used to analyze the N production well water temperature change curves, and the optimal production well water temperature change curve is selected. Based on the process parameters represented by the optimal production well water temperature change curve, thermal energy is extracted from the target dry hot rock reservoir.

2. The method for extracting thermal energy from deep fractured-bedrock interbedded dry hot rock as described in claim 1, characterized in that: When establishing the discrete fracture network model in step two, the fractures are treated as linear units with no thickness, and the deep high-temperature dry hot rock is discretized using solid units. The fracture length distribution of the fracture network usually adopts a power-law distribution model, and the relationship between the number of fractures and their length is expressed as (1-1): In the formula, The size of the field; The length of the crack; For a square reservoir with side length L, the fracture length l is given by... The number of cracks inside; It is the fractal dimension; This is the crack length index; It is a constant related to the crack density; and These represent the maximum and minimum fracture lengths within a square reservoir, respectively.

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