A heat recovery process control method for alleviating blockage of fractures in carbonate rock geothermal reservoirs

Abstract

The present invention discloses a heat extraction process control method for alleviating the blockage of fractures in carbonate-type geothermal reservoirs, belonging to the technical field of deep geothermal energy development and utilization. The method comprises: determining the scope of the simulation area, boundary conditions, and model parameters, and establishing a site artificial fracture structure model; considering the chemical reaction between the heat extraction fluid and the reservoir rock mass, constructing a mathematical model of the heat extraction process of the artificial fracture-type thermal reservoir; using the injection flow rate and the pH value of the injected fluid as input data, calculating the fracture permeability, the extraction well temperature, and the heat extraction amount; comparing the degree of fracture structure blockage and the system heat extraction amount under different injection flow rates and injection fluid pH values, and determining the preferred injection strategy with the goal of minimizing the blockage degree and maximizing the heat extraction amount; and adopting the preferred injection strategy in the actual geothermal energy extraction process. The present invention can alleviate the blockage of fractures in deep geothermal reservoirs and improve heat extraction efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of deep geothermal energy development and utilization, and specifically relates to a heat extraction process control method for alleviating the blockage of fractures in carbonate rock geothermal reservoirs. Background Art

[0002] Deep geothermal energy refers to geothermal energy stored in reservoirs with temperatures exceeding 150°C and buried deeper than 1.5 km. Due to its vast reserves and widespread distribution, it is a clean, renewable energy source that my country is prioritizing research and development. However, deep geothermal reservoirs generally suffer from poor permeability and poor hydrothermal circulation in their native state. This requires the creation of a fracture network with good hydraulic connectivity within the reservoir through artificial hydraulic fracturing and chemical stimulation to form an enhanced geothermal system, enabling the capture of deep geothermal energy.

[0003] Because geothermal reservoirs are exposed to high temperature and high pressure, even minor perturbations in the physical and chemical fields during the heat extraction process can cause significant changes in the fracture structure and affect the efficiency of heat exchange between injection and production wells. Deep geothermal energy has traditionally been extracted under conditions of constant flow, temperature, and the chemical composition of the injected fluid. Reports have indicated that, among the 66 enhanced geothermal systems constructed worldwide, reservoir fracture structures face varying degrees of chemical blockage risk under traditional heat extraction methods, with related research primarily focusing on granite-based geothermal reservoirs. However, for the more widespread carbonate-based geothermal reservoirs, chemical blockage under system operating conditions is more pronounced due to the more active rock chemistry. Currently, there is a lack of effective technologies and methods to suppress chemical blockage in carbonate-based geothermal reservoirs during the extraction phase. Summary of the Invention

[0004] The purpose of the present invention is to solve the problem of fissure blockage that is prone to occur during deep geothermal energy extraction in carbonate rock reservoirs, and to provide a more efficient and sustainable operation solution for deep geothermal development and utilization systems.

[0005] According to one aspect of the present invention, a heat recovery process control method for alleviating the blockage of fractures in carbonate rock-type geothermal reservoirs is provided, comprising: determining the scope of a simulation area, boundary conditions, and model parameters, and establishing a site artificial fracture structure model; constructing a mathematical model of the heat recovery process of the artificial fracture-type geothermal reservoir taking into account the chemical reaction between the heat recovery fluid and the reservoir rock mass; based on the mathematical model of the heat recovery process, taking the injection flow rate and the pH value of the injected fluid as input data, calculating the fracture permeability, the production well temperature, and the heat recovery amount, wherein the permeability reflects the degree of fracture blockage, and the heat recovery amount reflects the heat recovery capacity of the geothermal system; comparing the degree of fracture structure blockage and the system heat recovery amount under different injection flow rates and injected fluid pH values, and determining a preferred injection strategy with the goal of minimizing the degree of blockage and maximizing the heat recovery amount; and adopting the preferred injection strategy in the actual geothermal energy recovery process to alleviate the degree of fracture blockage in the carbonate rock-type geothermal reservoir and improve the heat recovery amount.

[0006] Optionally, the simulation area range, boundary conditions and model parameters are determined, and the establishment of the artificial fracture structure model of the site includes: determining the range of the model simulation area: the simulation area contains four fractures, the length and width of the fractures are between 500 and 600 m, and the simulation area range is 100 to 200 m laterally and vertically from the location of the fracture; after the expansion, the lateral range of the simulation area is 1200 m, and the vertical depth range is 4300 to 4850 m underground; determining the model boundary conditions: the outer boundary of the model is set as a water-proof boundary, and the top and bottom boundaries of the model are set as constant temperature boundaries, and the temperature is fixed at the average temperature of the reservoir at 150°C; determining the model parameters: the fracture opening of the model is set to 0.5 mm, the porosity is set to 0.8; the matrix pore permeability and porosity are set to 10 -13 m 2 and 0.005; the specific heat capacities of water and dolomite matrix are 4200 J / (kg·K) and 900 J / (kg·K), respectively, and the densities are 1000 kg / m 3 and 3000 kg / m 3 , and the thermal conductivity coefficients are 0.6 W / (m·K) and 1.0 W / (m·K), respectively.

[0007] Optionally, after determining the simulation area range, boundary conditions and model parameters and establishing the site artificial fracture structure model, it also includes analyzing the hydrothermal evolution law of the reservoir under mining conditions and adjusting the model parameters. The adjustment of the model parameters includes: when the model simulation runs for 5 years, the fracture porosity decreases from the initial 0.8 to 0.65, and the maximum blockage distance is 300 m; when the model simulation runs for 20 years, the blockage range increases to 450 m, the fracture porosity decreases to 0.61, and the seepage capacity of the fracture is reduced by 20 times.

[0008] Optionally, considering the chemical reaction between the heat recovery fluid and the reservoir rock, constructing a mathematical model for the heat recovery process of an artificial fractured geothermal reservoir includes: constructing the governing equations for the seepage process between the rock matrix and the fractures:

[0009] ;

[0010] Where, is the crack opening, is the fluid density, For time, represents the spatial gradient, is the seepage velocity, is the flow rate at the injection and production well location;

[0011] The governing equation for heat flow in the rock matrix and fractures is expressed as:

[0012] ;

[0013] Where, represents the overall specific heat capacity of the porous medium, Indicates temperature, represents the specific heat capacity of the fluid, represents the thermal conductivity of porous media;

[0014] Fracture opening and fracture permeability Satisfy between:

[0015] ;

[0016] Based on the calculation of seepage and heat flow processes, the migration of chemical components with the fluid follows the convection-diffusion equation:

[0017] ;

[0018] Where, For the The concentration of the ions, is the seepage velocity obtained by solving the governing equation of the seepage process. is the three-dimensional percolation space, is the diffusion coefficient, represents the change in solution concentration caused by chemical reaction and injection production, where

[0019] ;

[0020] Where, represents the injection intensity, is the effective porosity, and Proportional, is the initial concentration;

[0021] The chemical reaction process at different locations conforms to thermodynamic equilibrium, and its governing equation is:

[0022] ;

[0023] Where, It is The concentration of chemical components, Is related to temperature The associated equilibrium constant, and is the thermodynamic activity coefficient, is the number of ionic components, is the stoichiometric number.

[0024] Optionally, based on the heat recovery process mathematical model, with the injection flow rate and the pH value of the injected fluid as input data, calculating the fracture permeability and the production well temperature and the heat recovery includes: using the finite element method to solve the seepage control equation to obtain the seepage velocity ; Substitute the solved seepage velocity into the heat flow migration control equation to calculate the temperature spatial distribution; According to the solved temperature and seepage velocity, use them for convection diffusion equation and chemical reaction process calculation to obtain the concentration spatial distribution, and update the fracture opening and permeability for solving the seepage process control equation of the next time step; Repeat the above steps to calculate the fracture permeability and heat production calculation results of each time step, where the heat production is the temperature of the production well and traffic The product of .

[0025] Alternatively, the degree of fracture blockage and system heat production under different injection flow rates and injection fluid pH values ​​were compared. With the goal of minimizing the degree of blockage and maximizing the heat production, the optimal injection strategy was determined, including: using the constructed model, adjusting the injection fluid pH value under fixed injection flow conditions, alternately injecting conventional fluid with a pH value of 7.71 for 23 months, followed by injecting acidizing fluid with a pH value of 3.0 for 1 month. After 10 years of simulation operation, the simulation results showed that the fracture permeability at the production well location was 4.54×10 -9 m², while the fracture permeability of conventional fluid injection at a constant pH value is 4.36×10 -9 m²; using the constructed model, the injection flow rate was adjusted under the condition of fixed injection fluid pH value, and the injection flow rate was alternately 5 kg / s for 2 years, and then 10 kg / s for 2 years. The simulation results showed that when the injection method of alternating variable flow rate was used, the porosity and fracture permeability were 0.64 and 3.35×10 -9m², with a constant injection rate of 5 kg / s, the porosity and fracture permeability are 0.63 and 2.91×10 -9 m²; determine that periodically increasing the injection flow rate and lowering the pH value of the injected fluid is the preferred injection strategy.

[0026] The present invention restores the geological conditions of the actual geothermal reservoir by establishing a site model, constructs a mathematical model of the heat recovery process to analyze the physical and chemical changes involved in the heat recovery process, uses injection flow rate and pH value of the injected fluid as input data, analyzes the changing trends of fracture permeability and heat recovery under different injection parameters, and determines the optimal injection strategy with the goal of minimizing blockage and maximizing heat recovery to alleviate the blockage degree of fractures in carbonate rock geothermal reservoirs and improve heat recovery. This provides a more efficient and sustainable solution for underground fluid injection systems, has broad application prospects, can be used in the field of deep geothermal energy development, and can also be promoted and applied to other fields such as oil and gas development and carbon dioxide geological storage.

[0027] The present invention reduces the reduction in porosity and permeability by regulating the pH value and flow rate of the injected fluid and adopts a fluid injection method with variable flow rate and variable pH value, effectively alleviating the problem of blockage in the fracture structure of carbonate rock geothermal reservoirs, thereby extending the service life of the geothermal system and meeting the needs of long-term stable operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0029] Figure 1 This is a flow chart of a heat recovery process control method for alleviating blockage of fractures in a carbonate rock geothermal reservoir according to an embodiment of the present invention;

[0030] Figure 2 Schematic diagram of the artificial fissure structure model of the site in an embodiment of the present invention;

[0031] Figure 3 This is a distribution diagram of artificial fracture permeability under different injection flow conditions in an embodiment of the present invention;

[0032] Figure 4 This is a diagram showing the effect of applying the reservoir maintenance method on heat production capacity in an embodiment of the present invention. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. The described embodiments are only embodiments of a part of the present application, not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work should fall within the scope of protection of this application. It should be noted that, in the absence of conflict, the embodiments in the present application and the features in the embodiments can be combined with each other.

[0034] The terms "comprises" and "having" and any variations thereof in the specification and claims of this application are intended to cover non-exclusive inclusions. For example, a process, method, product or apparatus that includes a series of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, product or apparatus.

[0035] Reference Figure 1 , Figure 1 This is a flow chart of a heat extraction process control method for alleviating blockage of fractures in carbonate rock geothermal reservoirs according to an embodiment of the present invention. Figure 1 As shown, the method includes the following steps:

[0036] S1, determine the simulation area, boundary conditions and model parameters, and establish the artificial fracture structure model of the site;

[0037] The scope of the simulation area is reasonably defined based on the geological characteristics of the actual geothermal reservoir, mining planning and other factors; boundary conditions include temperature, pressure, fluid flow boundary, etc.; model parameters include geometric parameters of fractures (such as fracture opening, strike, inclination, etc.), physical properties of rocks (such as porosity, permeability), thermodynamic parameters (such as thermal conductivity, specific heat capacity), etc.

[0038] Figure 2 This is a schematic diagram of a site artificial fissure structure model in one embodiment of the present invention. Figure 2As shown in the figure, based on the geological structure, stratum, mineral composition, fracture direction, connectivity, and spatial distribution data of microseismic induced by fracturing, it is determined that the model contains four fractures with length and width ranging from 500 to 600 m. The simulation area extends 100 to 200 m from the location of the fracture to the periphery (including lateral and vertical directions). After the expansion, the lateral range of the simulation area is 1200 m, and the vertical depth range is 4300 to 4850 m underground. The outer boundary of the model is set as a water-proof boundary, and the fluid cannot penetrate through the boundary, ensuring the sealing of the fluid inside the model, so that the simulation process can focus on studying the fluid movement and heat transfer inside the simulation area without being disturbed by external fluid exchange. The top and bottom boundaries of the model are set as constant temperature boundaries, and the temperature is fixed at the average temperature of the reservoir at 150°C. The setting of the constant temperature boundary simulates the relatively stable temperature environment at the top and bottom of the actual geothermal reservoir, providing a stable temperature reference for the model, so that the model can more accurately simulate the transfer and exchange process of geothermal heat in the reservoir. The fracture opening of the model is set to 0.5 mm, porosity is set to 0.8, the fracture opening describes the size of the fracture space and affects the flow ability of the fluid in the fracture, and the porosity reflects the proportion of the space available for fluid in the fracture. The reasonable setting of these two parameters can accurately simulate the permeability characteristics of the fracture; the matrix pore permeability and porosity are set to 10 -13 m 2 and 0.005; the specific heat capacities of water and dolomite matrix are 4200 J / (kg·K) and 900 J / (kg·K), respectively, and the densities are 1000 kg / m 3 and 3000 kg / m 3 , and the thermal conductivity coefficients are 0.6 W / (m·K) and 1.0 W / (m·K), respectively.

[0039] Furthermore, after setting the above model parameters, the method of the present invention further includes analyzing the hydrothermal evolution of the reservoir under mining conditions and adjusting the above initially set model parameters after running for a specific period of time. Specifically:

[0040] During deep geothermal energy extraction, hot water is pumped out through the production wells, capturing heat. Cooled water is then injected into the reservoir, where it diffuses continuously from the injection wells to the production wells, forming a closed system. Due to the inconsistency between the composition of the reinjected fluid and the reservoir's native fluid, as well as the drop in temperature, calcite and gypsum precipitate. This precipitation exceeds the amount of dolomite that can be dissolved, leading to fissure blockage. This blockage worsens with continued system operation.

[0041] Under constant injection-production conditions (assuming the fluid migration capacity of the fractured reservoir meets the design flow rate and that there are no overpressure or dewatering issues at the wellhead), increased fracture permeability increases the sweep of the injected cold water perpendicular to the injection-production line (laterally), while its migration capacity in the direction of the injection-production line decreases. To maintain the injection rate, bottomhole pressure at the injection well increases.

[0042] After five years of operation, mineral blockage around the injection wells caused the fracture porosity to drop from an initial value of 0.8 to approximately 0.65, with the maximum blockage distance reaching approximately 300 meters. As the geothermal system continued to operate, the blockage radius increased to 450 meters, reaching its peak around the injection wells, where the porosity dropped to 0.61. This change in porosity reduced the fracture's seepage capacity (expressed as fracture permeability) by approximately 20 times.

[0043] S2, considering the chemical reaction between the heat recovery fluid and the reservoir rock, a mathematical model of the heat recovery process of the artificial fractured thermal reservoir is constructed;

[0044] Carbonate rocks are chemically active. During geothermal system operation, changes in reservoir temperature and the chemical compatibility of the injected fluid with the reservoir fluid can lead to mineral dissolution and precipitation, causing structural changes within the fractures and, in turn, affecting the system's efficiency. Therefore, based on a hydrothermal coupling model, a mathematical model was established and solved, taking into account the possible chemical reactions between the heat extraction fluid and the reservoir rock. Specifically, S2 includes:

[0045] S21, construct the governing equations for the seepage process in the rock matrix and fractures:

[0046]

[0047] The governing equation for heat flow in the rock matrix and fractures is expressed as:

[0048]

[0049] Where, is the crack opening, is the fluid density, For time, represents the spatial gradient, is the seepage velocity, is the flow rate at the injection and production well, represents the specific heat capacity of the porous medium as a whole (including solid and liquid), Indicates temperature, represents the specific heat capacity of the fluid, represents the thermal conductivity of porous media;

[0050] Crack opening and fracture permeability ( ) satisfy:

[0051] .

[0052] Based on the calculation of seepage and heat flow processes, the migration of chemical components with the fluid follows the convection-diffusion equation:

[0053] ;

[0054] Where, For the The concentration of the ions, It is the seepage velocity obtained by solving the seepage process control equation. is the three-dimensional percolation space, is the diffusion coefficient. Represents the change in solution concentration caused by chemical reaction and injection production:

[0055] ;

[0056] represents the injection intensity, is the effective porosity ( proportional to is the initial concentration; the chemical reaction process at different locations conforms to thermodynamic equilibrium, and its control equation is:

[0057] ;

[0058] Where, It is The concentration of chemical components, and is the thermodynamic activity coefficient, is the equilibrium constant (temperature dependent), is the number of ionic components, is the stoichiometric number.

[0059] S3, based on the mathematical model of the heat recovery process, with the injection flow rate ( ), pH value of injected fluid ( , , i.e. the concentration of the first ion - hydrogen ion) as input data, calculate the fracture permeability and the temperature and heat production of the production well. The permeability reflects the degree of fracture blockage, and the heat production reflects the heat production capacity of the geothermal system.

[0060] The degree of blockage is measured by the reduction in fracture permeability. A reduction in permeability indicates increased fracture blockage. The heat output reflects the heat extraction capacity of the geothermal system and is calculated by multiplying the well temperature and flow rate. Specifically, S3 includes:

[0061] S31, using the finite element method to solve the seepage control equation, discretize the complex seepage area into a finite number of small units, and approximately solve the seepage velocity in each unit to obtain the seepage velocity ( );

[0062] S32, substituting the solved seepage velocity into the heat flow migration control equation to calculate the temperature spatial distribution;

[0063] S33, based on the solved temperature and seepage velocity, is used to calculate the convection-diffusion equation and chemical reaction process, obtain the concentration spatial distribution, and update the fracture aperture and permeability for solving the seepage process control equation for the next time step;

[0064] Repeat the above S31~S33 process to obtain the calculation results of fracture permeability and heat production (product of well temperature and flow rate) for each time step ( Figure 3 and Figure 4 ).

[0065] The above calculation process is also known as the sequential coupled solution method. Simulations can be performed using COMSOL Multiphysics software, taking into account the actual site conditions. COMSOL Multiphysics is a multi-physics simulation software that supports the finite element method and can easily couple multiple physical fields such as seepage, heat transfer, and chemical reactions. It is suitable for complex geothermal reservoir simulation problems. This software allows for more intuitive observation and analysis of the operating status of geothermal systems and optimization of heat extraction control methods.

[0066] S4: Compare the degree of fracture blockage and system heat production under different injection flow rates and injected fluid pH values, and determine the optimal injection strategy with the goal of minimizing the blockage degree and maximizing the heat production;

[0067] By simulating the effects of human-controllable factors such as injection flow rate and pH value of injected fluid on the blockage degree of fracture structure and heat production efficiency, we draw a permeability distribution map and a curve of heat production changing with time, intuitively analyze the blockage degree of fracture and system heat production under different conditions, and determine the optimal injection strategy.

[0068] like Figure 3 The permeability distribution diagram of artificial fractures under different injection flow conditions is shown in Figure 2. Figure 3 It can be seen intuitively that when the injection flow rate is 30 kg / s, the reduction in permeability is the smallest, indicating that under this flow condition, the degree of fracture blockage is relatively light, that is, the fracture structure is maintained (the degree of blockage is alleviated), thus determining 30 kg / s as the optimal injection flow rate.

[0069] Furthermore, in actual geothermal projects, constant pressure injection and constant pressure extraction are usually adopted. Under these conditions, chemical blockage of reservoir fissures may cause a significant reduction in flow rate. The present invention uses the constructed model to adjust the pH value of the injected fluid under the condition of a fixed injection flow rate, and to adjust the injection flow rate under the condition of a fixed injected fluid pH value. The degree of fissure structure blockage and the system heat extraction are compared under the conditions of variable pH value and constant pH value, and variable flow rate and constant flow rate, respectively. It is concluded that the strategy of periodically changing flow rate and changing fluid pH value is the preferred injection strategy. Specifically:

[0070] (1) pH control under fixed flow conditions:

[0071] First, under constant flow conditions, conventional fluid (pH = 7.71) was injected for 23 months, followed by injection of acidified fluid (pH = 3.0) for 1 month. By comparing with the injection at a constant pH value, the permeability at the production well location was 4.54×10 -9 m², while the value when conventional fluid is injected is 4.36×10 -9 m².

[0072] (2) Variable flow injection method:

[0073] A variable flow injection strategy was further adopted, with injection at a rate of 5 kg / s for two years, followed by injection at a rate of 10 kg / s for two years, and so on. Simulation results showed that compared with constant flow injection (5 kg / s), the range and magnitude of permeability reduction under variable flow injection conditions were significantly reduced. Although the temperature of the production well decreased, the increase in cumulative flow rate can improve the overall heat recovery efficiency of the system. For example, after 10 years of system operation, the porosity and permeability under constant flow injection conditions were 0.63 and 2.91×10 -9 m², and under variable flow injection conditions, the values ​​increased to 0.64 and 3.35×10 -9 m², greater permeability and better reservoir maintenance effect.

[0074] In summary, periodically increasing the injection flow rate (from 5kg / s to 10kg / s) and lowering the fluid pH (from 7.71 to 3.0) are key to mitigating fracture blockage and improving system efficiency. This is because the acidizing fluid can chemically react with and dissolve any sediment that may form within the fractures. For example, when the geothermal fluid contains easily precipitated materials such as carbonates, the hydrogen ions in the acidizing fluid react with the carbonates to form soluble ions, thereby reducing the accumulation of sediment within the fractures and effectively alleviating fracture blockage. Variable flow rate, on the other hand, alters the fluid flow velocity and pressure distribution within the fractures. By varying the injection velocity and pressure, it makes it difficult for sediment to accumulate locally, reducing sediment deposition within the fractures and further alleviating fracture blockage.

[0075] S5, adopting the preferred injection strategy in the actual geothermal energy extraction process to alleviate the blockage degree of the carbonate rock geothermal reservoir fissures and improve the heat extraction.

[0076] By regulating the pH value and flow rate of the injected fluid, the present invention alleviates the problem of blockage in the fracture structure of carbonate-type geothermal reservoirs, providing a more efficient and sustainable solution for underground fluid injection systems. Furthermore, because the conditions in actual geothermal reservoirs are more complex than those simulated by the model, when applying the preferred injection strategy to the actual geothermal energy extraction process, adaptive adjustments can be made based on the actual situation on site. For example, a comprehensive monitoring system can be established to monitor parameters such as porosity, fracture blockage, and system heat extraction in real time, and the model parameters and injection strategy can be adjusted in a timely manner based on the monitoring results.

[0077] This paper proposes an innovative method for optimizing deep geothermal energy development systems. By regulating the pH value and flow rate of the injected fluid, it effectively alleviates the blockage problem of fracture structures in carbonate-type geothermal reservoirs and significantly improves the heat extraction efficiency of the geothermal system. Specific effects include:

[0078] 1. The reduction in fracture permeability is reduced ( Figure 3 ), thereby extending the system operating life;

[0079] 2. As the reservoir fracture opening is maintained, according to the actual project operation conditions, under the condition of fixed injection pressure, the heat production temperature and flow rate of the whole system production well can be improved, thereby promoting the increase of the system heat production ( Figure 4 , Figure 4 It shows that regardless of whether the evolution of the fracture structure is considered or whether fracture structure maintenance is implemented, the heat recovery is improved after the present invention is used to alleviate the degree of fracture blockage and maintain the reservoir effect), which is suitable for the needs of long-term stable operation.

[0080] The present invention provides a more efficient and sustainable solution for underground fluid injection systems. It has broad application prospects and can be used in deep geothermal energy development, as well as other fields such as oil and gas development and carbon dioxide geological storage.

[0081] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A heat recovery process control method for alleviating blockage of fractures in carbonate rock geothermal reservoirs, characterized in that: include: Determine the simulation area, boundary conditions and model parameters, and establish the artificial fracture structure model of the site; Considering the chemical reaction between the heat recovery fluid and the reservoir rock, a mathematical model of the heat recovery process in artificial fractured geothermal reservoirs is constructed; Based on the mathematical model of the heat recovery process, the injection flow rate and the pH value of the injected fluid are used as input data to calculate the fracture permeability, the production well temperature and the heat recovery rate, wherein the permeability reflects the degree of fracture blockage and the heat recovery rate reflects the heat recovery capacity of the geothermal system; Compare the degree of fracture blockage and system heat production under different injection flow rates and injected fluid pH values, and determine the optimal injection strategy with the goal of minimizing blockage and maximizing heat production; The preferred injection strategy is applied in the actual geothermal energy extraction process to alleviate the blockage degree of fractures in carbonate rock geothermal reservoirs and improve the heat extraction; Considering the chemical reaction between the heat recovery fluid and the reservoir rock, the mathematical model of the heat recovery process of artificial fractured geothermal reservoirs is constructed, including: Construct the governing equations for the seepage process in the rock matrix and fractures: ； Where, is the crack opening, is the fluid density, For time, represents the spatial gradient, is the seepage velocity, is the flow rate at the injection and production well location; The governing equation for heat flow in the rock matrix and fractures is expressed as: ； Where, represents the overall specific heat capacity of the porous medium, Indicates temperature, represents the specific heat capacity of the fluid, represents the thermal conductivity of porous media; Crack opening and fracture permeability Satisfy between: ； Based on the calculation of seepage and heat flow processes, the migration of chemical components with the fluid follows the convection-diffusion equation: ； Where, For the The concentration of the ions, It is the seepage velocity obtained by solving the seepage process control equation. is the three-dimensional percolation space, is the diffusion coefficient, represents the change in solution concentration caused by chemical reaction and injection production, where ； Where, represents the injection intensity, is the effective porosity, and Proportional, is the initial concentration; The chemical reaction process at different locations conforms to thermodynamic equilibrium, and its governing equation is: ； Where, It is The concentration of chemical components, Is related to temperature The associated equilibrium constant, and is the thermodynamic activity coefficient, is the number of ionic components, is the stoichiometric number.

2. The heat recovery process control method for alleviating blockage of fractures in carbonate rock geothermal reservoirs according to claim 1, characterized in that: Determine the simulation area, boundary conditions, and model parameters, and establish the artificial fracture structure model of the site, including: Determine the scope of the model simulation area: The simulation area contains four fractures with lengths and widths ranging from 500 to 600 meters. The simulation area extends 100 to 200 meters laterally and vertically from the fracture location. After expansion, the lateral range of the simulation area is 1200 meters, and the vertical depth range is 4300 to 4850 meters underground. Determine the model boundary conditions: the outer boundary of the model is set as a water-proof boundary, and the top and bottom boundaries of the model are set as constant temperature boundaries, with the temperature fixed at the average temperature of the reservoir, 150°C; Determine the model parameters: the fracture opening of the model is set to 0.5 mm, the porosity is set to 0.8; the matrix pore permeability and porosity are set to 10 -13 m 2 and 0.005; the specific heat capacities of water and dolomite matrix are 4200 J / (kg·K) and 900 J / (kg·K), respectively, and the densities are 1000 kg / m 3 and 3000 kg / m 3 , and the thermal conductivity coefficients are 0.6 W / (m·K) and 1.0 W / (m·K), respectively.

3. The heat recovery process control method for alleviating blockage of fractures in carbonate rock geothermal reservoirs according to claim 2, characterized in that: After determining the simulation area, boundary conditions, and model parameters, and establishing the artificial fracture structure model of the site, the process also includes analyzing the hydrothermal evolution of the reservoir under mining conditions and adjusting the model parameters. The adjustment of the model parameters includes: After 5 years of model simulation, the fracture porosity decreased from the initial 0.8 to 0.65, and the maximum blockage distance was 300 m; After 20 years of model simulation, the blockage range increased to 450 m, the fracture porosity decreased to 0.61, and the fracture seepage capacity decreased by 20 times.

4. The heat recovery process control method for alleviating blockage of fractures in carbonate rock geothermal reservoirs according to claim 1, characterized in that: Based on the mathematical model of the heat recovery process, the injection flow rate and the pH value of the injected fluid are used as input data to calculate the fracture permeability, the production well temperature and the heat recovery amount, including: S1: Use the finite element method to solve the seepage control process equation to obtain the seepage velocity ; S2: Substituting the solved seepage velocity into the heat flow migration control equation to calculate the temperature spatial distribution; S3: The temperature and seepage velocity obtained are used to calculate the convection-diffusion equation and chemical reaction process, obtain the concentration spatial distribution, and update the fracture opening and permeability for solving the seepage process control equation for the next time step; S4: Repeat the steps in S1 to S3 to calculate the fracture permeability and heat production results for each time step, where the heat production is the temperature of the production well. and traffic The product of .

5. The heat recovery process control method for alleviating blockage of fractures in carbonate rock geothermal reservoirs according to claim 4, characterized in that: By comparing the degree of fracture blockage and system heat production under different injection flow rates and injected fluid pH values, and with the goal of minimizing blockage and maximizing heat production, the optimal injection strategy was determined, including: The constructed model was used to adjust the pH value of the injected fluid under the condition of fixed injection flow rate. Conventional fluid with a pH value of 7.71 was injected alternately for 23 months, followed by acidizing fluid with a pH value of 3.0 for 1 month. The simulation results showed that the fracture permeability at the production well location was 4.54×10- -9 m², while the fracture permeability of conventional fluid injection at a constant pH value is 4.36×10 -9 m²; The constructed model was used to adjust the injection flow rate under the condition of fixed injection fluid pH value. The injection flow rate was alternately 5 kg / s for 2 years and then 10 kg / s for 2 years. The simulation results showed that the porosity and fracture permeability of the alternating injection flow rate were 0.64 and 3.35×10-1 respectively after 10 years of simulation. -9 m², with a constant injection rate of 5 kg / s, the porosity and fracture permeability are 0.63 and 2.91×10 -9 m²; It is determined that periodically increasing the injection flow rate and reducing the pH value of the injected fluid is the preferred injection strategy.

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