Method and system for determining convection heat effect of karst type geothermal system

Abstract

The invention discloses a method and system for determining the convection heat effect of a karst-type geothermal system, and the method comprises the steps: measuring thermophysical parameters of a rock sample from a target region, converting the thermophysical parameters into a cylindrical coordinate system, forming a rock thermophysical column, constructing a geothermal geologic model, and based on the thermophysical parameters, combining the flow characteristics and heat transfer characteristics of underground water in the target region, calculating the convection heat effect of the karst-type geothermal system. Constructing a geothermal geology mathematical model; only configuring the thermal boundary condition to the mathematical model to obtain the thermal flux at the boundary of the model; meanwhile, the thermal boundary condition and the constant pressure boundary condition are configured in a geothermal geology mathematical model, the heat flux is combined, the heat gathering effect of free convection at the boundary of the model is obtained as the free convection heat effect, and the thermal boundary condition, the constant pressure boundary condition and the water head boundary condition are further configured in the mathematical model at the same time; and in combination with the heat-gathering effect of free convection, obtaining the heat-gathering effect of forced convection at the boundary of the model as the heat-gathering effect of forced convection. The quantitative research on the heat gathering mechanism of the karst type geothermal system is realized.

Description

Technical Field

[0001] This invention belongs to the field of geothermal resource development technology, and in particular relates to a method and system for determining the convective heat effect of karst geothermal systems. Background Technology

[0002] Geothermal resources are a highly competitive clean and renewable energy source with many advantages, such as large resource volume, high energy efficiency, low cost, and good energy conservation and emission reduction effects. It is one of the important alternative new energy sources in the future.

[0003] Karst geothermal reservoirs refer to geothermal reservoirs composed of karstified carbonate rocks (limestone, dolomite, marble, etc.), sulfate rocks (gypsum, anhydrite, mirabilite, etc.), and halide rocks (rock salt, potash, magnesium salt, etc.). Due to the development of karst fissures, karst geothermal reservoirs possess excellent vertical seepage and horizontal runoff conditions, resulting in large water yields and easy reinjection of tailwater after geothermal utilization. Therefore, they are the most promising and dominant reservoir types for development.

[0004] In karst geothermal systems, rapid heat transfer via fluid convection creates significant local thermal anomalies, forming "sweet spots" for geothermal field exploration and development. Therefore, quantitatively evaluating the heat accumulation effect of groundwater convection is of particular significance for the geological evaluation and genetic mechanism research of karst geothermal reservoirs. In karst reservoirs, geological and hydrodynamic conditions are complex, and the deep temperature field is influenced by the anisotropy of the underground medium and fluid activity. Furthermore, the fluid flow system evolves over time, with topographically driven forced convection and buoyancy-driven natural thermal convection often acting together on the geothermal system. However, due to limitations in research methods and scale, existing studies on the temperature field structure and heat accumulation mechanism of karst reservoirs are still mainly qualitative analyses, lacking quantitative research on the heat accumulation mechanism of complex convection-conduction geothermal systems. Furthermore, in the few quantitative studies using numerical simulations, the research scale is often limited to a single secondary tectonic unit or geothermal field, lacking research on the heat accumulation mechanism of groundwater flow systems at the watershed scale. In particular, there is a lack of research on the combined effects of topographically driven forced convection and buoyancy-driven natural convection, and their heat accumulation effects. Therefore, a method is needed to quantitatively assess the thermal effects of forced and natural convection in karst geothermal reservoirs, thereby scientifically understanding the heat accumulation mechanism of karst geothermal systems and determining the basis for the placement of high-temperature, high-yield geothermal wells in karst reservoirs. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for determining the convective heat effect of a karst geothermal system, comprising: measuring the thermal property parameters of rock samples taken from a target area and converting them to a cylindrical coordinate system to form a rock thermal property column; constructing a geothermal geological model using the rock thermal property column, and based on this, constructing a geothermal geological mathematical model by combining the flow characteristics and heat transfer characteristics of groundwater in the target area; configuring only the thermal boundary conditions in the mathematical model to obtain the heat flux at the model boundary; simultaneously configuring the thermal boundary conditions and the isobaric boundary conditions in the mathematical model, and combining the heat flux to obtain the heat accumulation effect of free convection at the model boundary as the free convection heat effect of the target area; further, simultaneously configuring the thermal boundary conditions, the isobaric boundary conditions, and the hydraulic head boundary conditions in the mathematical model, and combining the heat accumulation effect of free convection to obtain the heat accumulation effect of forced convection at the model boundary as the forced convection heat effect of the target area.

[0006] Preferably, the step of constructing a geothermal geological model using the rock thermal property column includes: dividing the target area into several rock stratigraphic units based on geophysical logging data and borehole data, and then integrating the rock thermal property parameters with each rock stratigraphic unit according to the division results, thereby constructing the geothermal geological model, wherein the geothermal geological model is a two-dimensional model or a three-dimensional model.

[0007] Preferably, the method further includes: by treating the target area as an equivalent porous medium and combining the variation characteristics of the physical parameters of groundwater with temperature, determining the coupling relationship between the temperature field and the seepage field within the target area, so as to obtain the flow characteristics and heat transfer characteristics of groundwater within the target area.

[0008] Preferably, the flow characteristics and heat transfer characteristics of groundwater within the target area are represented by the following expression:

[0009]

[0010] in, ρ represents the porosity of the equivalent porous medium. w The density of groundwater is represented by t, and time is represented by t. Let represent the gradient operator, q represent the groundwater velocity, K represent the permeability coefficient, and g represent the gravitational acceleration. The pressure gradient representing the direction of gravity. c represents the height gradient. w ρ represents the specific heat capacity of groundwater. ma c represents the density of porous media. ma This represents the specific heat capacity of the porous medium, where T represents temperature. λ represents the temperature gradient. wλ represents the thermal conductivity of groundwater. ma Q represents the thermal conductivity of porous media. r This indicates that the rock is radioactively generating heat.

[0011] Preferably, the variation characteristics of the physical parameters of the groundwater with temperature are represented by the following expression:

[0012] c w (T) = 12010.14 - 80.40T + 0.31T 2 -5.38×10 -4 T 3 +3.63×10 -7 T 4

[0013] μ w (T) = 0.004 - 2.108 × 10 -5 T+3.858×10 -8 T 2 -2.397×10 -11 T 3

[0014] ρ w (T) = 1002.4 - 0.1905T - 0.0025T 2

[0015] Where, μ w This indicates the dynamic viscosity of groundwater.

[0016] Preferably, the step of obtaining the heat flux at the model boundary includes: performing numerical simulation using a geothermal geological mathematical model configured with the thermal boundary conditions to obtain the geothermal gradient of the model under conduction heat transfer only, and then combining the geothermal gradient with the thermal conductivity of the model to calculate the heat flux at the model boundary.

[0017] Preferably, the step of obtaining the heat accumulation effect of free convection at the model boundary as the free convection thermal effect of the target area includes: performing a numerical simulation using the same simulation parameters as the geothermal geological mathematical model configured with the thermal boundary conditions, and using a geothermal geological mathematical model configured with both thermal and isobaric boundary conditions, thereby obtaining the heat flux at the model boundary under the combined action of conduction and fluid heat transfer, independent of topographic relief; and calculating the heat accumulation effect of free convection at the model boundary in conjunction with the heat flux of the geothermal geological mathematical model configured with the thermal boundary conditions.

[0018] Preferably, the step of obtaining the heat accumulation effect of forced convection at the model boundary as the forced convection thermal effect of the target area includes: performing numerical simulation using the same simulation parameters as the geothermal geological mathematical model that is simultaneously configured with the thermal boundary conditions, the constant pressure boundary conditions, and the hydraulic head boundary conditions, thereby obtaining the heat flux at the model boundary under the combined action of conduction heat transfer and fluid heat transfer, which is related to the topographic relief; and calculating the heat accumulation effect of forced convection at the model boundary in conjunction with the heat accumulation effect of free convection at the model boundary.

[0019] Preferably, after constructing the geothermal geological mathematical model, the method further includes: performing gridding processing on the constructed geothermal geological model, so as to obtain the actual heat flux at the model boundary, the heat accumulation effect of free convection at the model boundary, and the heat accumulation effect of forced convection at the model boundary using the gridded geothermal geological model.

[0020] On the other hand, the present invention also provides a system for determining the convective heat effect of a karst geothermal system. The system includes the following modules: a parameter acquisition module, which measures the thermal property parameters of rock samples taken from a target area and converts them to a cylindrical coordinate system to form a rock thermal property column; a model construction module, which uses the rock thermal property column to construct a geothermal geological model, and based on this, combines the flow characteristics and heat transfer characteristics of groundwater in the target area to construct a geothermal geological mathematical model; a first thermal effect acquisition module, which configures only the thermal boundary conditions in the mathematical model to obtain the heat flux at the model boundary; and a second thermal effect acquisition module, which configures the thermal boundary conditions and the isobaric boundary conditions in the mathematical model simultaneously, and combines the heat flux to obtain the free convection heat accumulation effect at the model boundary as the free convection heat effect of the target area, and further configures the thermal boundary conditions, the isobaric boundary conditions, and the hydraulic head boundary conditions in the mathematical model simultaneously, and combines the free convection heat accumulation effect to obtain the forced convection heat accumulation effect at the model boundary as the forced convection heat effect of the target area.

[0021] Compared with the prior art, one or more embodiments of the above solutions may have the following advantages or beneficial effects:

[0022] This invention provides a method and system for determining the convective heat effect of karst geothermal systems. The method first forms a rock thermal property column based on the thermal property parameters of rock samples taken from the target area. Then, a geothermal geological model is constructed based on the formed rock thermal property column, without considering the flow and heat transfer states of groundwater. On this basis, the flow and heat transfer characteristics of groundwater are then incorporated into the geothermal geological model to construct a geothermal geological mathematical model. Finally, for the geothermal geological mathematical model, thermal boundary conditions are configured only, thermal boundary conditions and isobaric boundary conditions are configured simultaneously, and thermal boundary conditions, isobaric boundary conditions, and hydraulic head boundary conditions are configured simultaneously. Ultimately, the heat accumulation effect of free convection at the model boundary is obtained as the free convection heat effect of the target area, and the heat accumulation effect of forced convection at the model boundary is obtained as the forced convection heat effect of the target area. This invention enables a quantitative study of the heat accumulation mechanism of karst geothermal systems, scientifically understanding the heat accumulation mechanism of geothermal systems, and providing a basis for determining the location of high-temperature, high-yield geothermal wells in karst geothermal reservoirs.

[0023] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the description, claims, and drawings. Attached Figure Description

[0024] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with the embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0025] Figure 1 This is a step diagram of a method for determining the convective heat effect of a karst geothermal system according to an embodiment of this application.

[0026] Figure 2 This is a block diagram of the system for determining the convective heat effect of a karst geothermal system according to an embodiment of this application. Detailed Implementation

[0027] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings and examples, so that the process of how the present invention uses technical means to solve technical problems and achieve technical effects can be fully understood and implemented accordingly. It should be noted that, as long as there is no conflict, the various embodiments and features in the various embodiments of the present invention can be combined with each other, and the resulting technical solutions are all within the protection scope of the present invention.

[0028] Furthermore, the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0029] Geothermal resources are a highly competitive clean and renewable energy source with many advantages, such as large resource volume, high energy efficiency, low cost, and good energy conservation and emission reduction effects. It is one of the important alternative new energy sources in the future.

[0030] Karst geothermal reservoirs refer to geothermal reservoirs composed of karstified carbonate rocks (limestone, dolomite, marble, etc.), sulfate rocks (gypsum, anhydrite, mirabilite, etc.), and halide rocks (rock salt, potash, magnesium salt, etc.). Due to the development of karst fissures, karst geothermal reservoirs possess excellent vertical seepage and horizontal runoff conditions, resulting in large water yields and easy reinjection of tailwater after geothermal utilization. Therefore, they are the most promising and dominant reservoir types for development.

[0031] In karst geothermal systems, rapid heat transfer via fluid convection creates significant local thermal anomalies, forming "sweet spots" for geothermal field exploration and development. Therefore, quantitatively evaluating the heat accumulation effect of groundwater convection is of particular significance for the geological evaluation and genetic mechanism research of karst geothermal reservoirs. In karst reservoirs, geological and hydrodynamic conditions are complex, and the deep temperature field is influenced by the anisotropy of the underground medium and fluid activity. Furthermore, the fluid flow system evolves over time, with topographically driven forced convection and buoyancy-driven natural thermal convection often acting together on the geothermal system. However, due to limitations in research methods and scale, existing studies on the temperature field structure and heat accumulation mechanism of karst reservoirs are still mainly qualitative analyses, lacking quantitative research on the heat accumulation mechanism of complex convection-conduction geothermal systems. Furthermore, in the few quantitative studies using numerical simulations, the research scale is often limited to a single secondary tectonic unit or geothermal field, lacking research on the heat accumulation mechanism of groundwater flow systems at the watershed scale. In particular, there is a lack of research on the combined effects of topographically driven forced convection and buoyancy-driven natural convection, and their heat accumulation effects. Therefore, a method is needed to quantitatively assess the thermal effects of forced and natural convection in karst geothermal reservoirs, thereby scientifically understanding the heat accumulation mechanism of karst geothermal systems and determining the basis for the placement of high-temperature, high-yield geothermal wells in karst reservoirs.

[0032] Therefore, to address the aforementioned problems, this invention proposes a method and system for determining the convective heat effect of karst geothermal systems. The method first forms a rock thermal property column based on the thermal property parameters of rock samples taken from the target area. Then, a geothermal geological model is constructed based on the formed rock thermal property column, without considering the flow and heat transfer states of groundwater. On this basis, the flow and heat transfer characteristics of groundwater are then incorporated into the geothermal geological model to construct a geothermal geological mathematical model. Finally, for the geothermal geological mathematical model, thermal boundary conditions are configured only, thermal boundary conditions and isobaric boundary conditions are configured simultaneously, and thermal boundary conditions, isobaric boundary conditions, and hydraulic head boundary conditions are configured simultaneously. Ultimately, the heat accumulation effect of free convection at the model boundary is obtained as the free convection heat effect of the target area, and the heat accumulation effect of forced convection at the model boundary is obtained as the forced convection heat effect of the target area. This invention achieves a quantitative study of the heat accumulation mechanism of karst geothermal systems, enabling a scientific understanding of the heat accumulation mechanism of geothermal systems and providing a basis for determining the location of high-temperature, high-yield geothermal wells in karst geothermal reservoirs.

[0033] Example 1

[0034] Figure 1 This is a flowchart illustrating the steps of a method for determining the convective heat effect of a karst geothermal system according to an embodiment of this application. See below for reference. Figure 1 This will explain each step of the method.

[0035] like Figure 1 As shown, in step S110, the thermal properties of rock samples taken from the target area are measured and converted to a cylindrical coordinate system to form a rock thermal property column. Specifically, in this embodiment, several rock samples are first collected from different locations within the target area of ​​the karst geothermal reservoir to ensure that the collected rock samples completely cover all rock stratigraphic units within the target area. Then, the thermal properties of each collected rock sample are measured, and each thermal property parameter obtained through measurement is converted to a cylindrical coordinate system according to its location within the target area. Based on this, by interpolating the rock thermal property parameters at locations other than the rock samples collected within the target area in the cylindrical coordinate system, a rock thermal property column representing the overall rock thermal property distribution characteristics of the target area is formed, thereby providing a data foundation for the construction of the geothermal geological model in the following text.

[0036] In one specific embodiment of this application, the thermophysical parameters include, but are not limited to: density, heat capacity, porosity, permeability, thermal conductivity, and heat generation rate.

[0037] Further, in step S120, a geothermal geological model is constructed using a rock thermal property column. Based on this, a geothermal geological mathematical model is constructed by combining the flow and heat transfer characteristics of groundwater in the target area. In this embodiment, a geological model (excluding thermal property parameters) is first established based on measured data from the target area, including borehole data, profiles, seismic data, isobaths, geological maps, topographic maps, geophysical data, geochemical data, engineering survey data, and hydrological monitoring data. This model reflects the geological structure, structural relationships, and internal property variation patterns of the geological body in the target area. Then, the rock thermal property column is loaded onto the geological model, thus completing the construction of the geothermal geological model. Next, the flow and heat transfer characteristics of groundwater in the target area are analyzed to obtain these characteristics. Finally, the flow and heat transfer characteristics of groundwater in the target area are loaded onto the geothermal geological model, thus completing the construction of the geothermal geological mathematical model.

[0038] In the process of constructing a geothermal geological model using rock thermal property columns, the target area is divided into several lithostratigraphic units based on geophysical logging and borehole data. Then, based on the division results, rock thermal property parameters are integrated with each lithostratigraphic unit to construct the geothermal geological model. For example, based on geophysical logging and borehole data of the target area, lithostratigraphic units are divided, including metamorphic basement, Middle-Upper Proterozoic Changcheng-Jixian carbonate rocks, Lower Paleozoic Cambrian-Ordovician carbonate strata, Mesozoic sandstone and mudstone (locally developed), Paleogene sandstone and mudstone, Neogene sandstone and mudstone, and Quaternary clay layers (i.e., several lithostratigraphic units). Among these, the tops of the Middle-Upper Proterozoic, Cambrian, and Ordovician carbonate rocks represent karst geothermal reservoirs. Next, the various thermal property parameters in the rock thermal property column are integrated with the corresponding stratigraphic units according to the corresponding stratigraphic depth. The combination of all the stratigraphic units carrying the rock thermal property parameters after integration constitutes the target area carrying the rock thermal property parameters, thus achieving the purpose of constructing a geothermal geological model.

[0039] In one specific embodiment of this application, the geothermal geological model is a two-dimensional model or a three-dimensional model.

[0040] Furthermore, this invention also determines the coupling relationship between the temperature field and the seepage field within the target area by treating the target area as an equivalent porous medium and combining the temperature-dependent physical parameters of groundwater, thereby obtaining the flow and heat transfer characteristics of groundwater within the target area. Specifically, in this embodiment, the rock strata are considered as porous media within the aquifer, meaning the target area is treated as an equivalent porous medium, and the heat transfer process can be described by the heat transfer through the porous medium. Accordingly, based on the conservation of mass, Darcy's law, and energy conservation (heat transfer through porous media), the flow process of groundwater is described using the fluid continuity equation and Darcy's law. By solving the equations describing the groundwater flow process, the flow and heat transfer characteristics of groundwater within the target area are obtained. Next, during the solution process, the coupling relationship between the temperature field and the seepage field within the target area is mainly obtained through the changes in the physical parameters of groundwater (e.g., density, dynamic viscosity, and specific heat capacity). That is, during the flow through areas with temperature differences, the physical parameters of groundwater no longer remain constant but change with temperature. Therefore, only by determining the coupling relationship between the temperature field and the seepage field within the target area can the aforementioned solution process be achieved, thereby obtaining the flow characteristics and heat transfer characteristics of groundwater within the target area.

[0041] Next, the flow and heat transfer characteristics of groundwater within the target area are represented by the following expression:

[0042]

[0043] in, ρ represents the porosity of the equivalent porous medium. w The density of groundwater is represented by t, and time is represented by t. Let represent the gradient operator, q represent the groundwater velocity, K represent the permeability coefficient, and g represent the gravitational acceleration. The pressure gradient representing the direction of gravity. c represents the height gradient. w ρ represents the specific heat capacity of groundwater. ma c represents the density of porous media. ma This represents the specific heat capacity of the porous medium, where T represents temperature. λ represents the temperature gradient. w λ represents the thermal conductivity of groundwater. ma Q represents the thermal conductivity of porous media. r This indicates that the rock is radioactively generating heat.

[0044] In one specific embodiment of this application, the temperature range of the area through which the groundwater flows, where there is a temperature difference, is 273–553 K. In this case, the specific heat capacity of the groundwater as a function of temperature is represented by the following expression:

[0045] cw (T) = 12010.14 - 80.40T + 0.31T 2 -5.38×10 -4 T 3 +3.63×10 -7 T 4 (4)

[0046] In one specific embodiment of this application, the temperature range of the area through which the groundwater flows is 273–553 K. In this case, the density of the groundwater as a function of temperature is represented by the following expression:

[0047] ρ w (T) = 1002.4 - 0.1905T - 0.0025T 2 (5)

[0048] In addition, the permeability coefficient varies with the dynamic viscosity and density of groundwater. The relationship between the permeability coefficient and the permeability rate is expressed by the following expression:

[0049]

[0050] Where k represents the penetration rate.

[0051] In one specific embodiment of this application, the temperature range of the area through which the groundwater flows is 273–553 K. In this case, the dynamic viscosity of the groundwater as a function of temperature is represented by the following expression:

[0052] μ w (T) = 0.004 - 2.108 × 10 -5 T+3.858×10 -8 T 2 -2.397×10 -11 T 3 (7)

[0053] Where, μ w This indicates the dynamic viscosity of groundwater.

[0054] After the geothermal geological mathematical model is constructed, different boundary conditions and initial values ​​are configured for it to obtain the convective heat effect of the target area using numerical simulation results under different boundary conditions and initial values. For example, the surface temperature of the mathematical model is set to the regional annual average temperature of 15℃ and it is an open boundary; the vertical temperature is set to 25℃ / km based on the local measured geothermal gradient; the basal heat flow is obtained from the surface heat flow value through the back-stripping method; and there is no longitudinal water flow in the basal region, and the two sides are adiabatic boundaries with no lateral water flow.

[0055] Furthermore, in step S130, only thermal boundary conditions are configured in the mathematical model to obtain the heat flux at the model boundary. Specifically, in this embodiment, only thermal boundary conditions are configured in the mathematical model to form a pure heat conduction model (M1) that does not consider the groundwater flow state. The heat flux at the model boundary is obtained by solving the pure heat conduction model (M1). For example, the thermal boundary conditions are set as follows: Dirichlet boundary conditions are set at the top boundary of the mathematical model (the upper surface of the mathematical model, usually referring to the ground surface), and the temperature value is the annual average temperature T0 of the target area; the bottom boundary of the mathematical model (the lower surface of the mathematical model) is set as Neumann boundary conditions, that is, a constant heat flux value is given, which is obtained by the back-stripping method based on the surface heat flux value; all boundaries of the mathematical model are adiabatic boundaries.

[0056] In the step of obtaining the heat flux at the model boundary, a geothermal geological mathematical model with thermal boundary conditions is used for numerical simulation to obtain the geothermal gradient of the model under conduction-only heat transfer. The geothermal gradient is then combined with the thermal conductivity of the model to calculate the heat flux at the model boundary. In this embodiment, a pure heat conduction model (M1) is used for numerical simulation, and solid heat transfer calculations are performed by solving the pure heat conduction model (M1) to obtain the temperature distribution pattern of the target area under the current simulation conditions. That is, without groundwater convection, the temperature distribution can be obtained from the initial temperature conditions according to the heat transfer equation. Based on this, the aforementioned temperature distribution pattern is analyzed to obtain the geothermal gradient of the model under conduction-only heat transfer. Then, by multiplying the geothermal gradient by the thermal conductivity of the model, the heat flux at the model boundary can be obtained (at this point, the heat flux at the model boundary essentially represents the geothermal heat flow).

[0057] Furthermore, in step S140, thermal boundary conditions and isobaric boundary conditions are simultaneously configured in the mathematical model, and combined with heat flux, the heat accumulation effect of free convection at the model boundary is obtained as the free convection heat effect of the target region. Furthermore, thermal boundary conditions, isobaric boundary conditions and head boundary conditions are simultaneously configured in the mathematical model, and combined with the heat accumulation effect of free convection, the heat accumulation effect of forced convection at the model boundary is obtained as the forced convection heat effect of the target region. In this embodiment, both thermal boundary conditions and isobaric boundary conditions are configured in the mathematical model to form a free convection model (M2) (i.e., the free convection model (M2) is a fluid convection model coupled with different driving mechanisms on the pure heat conduction model (M1)). By solving the free convection model (M2) and further using the pure heat conduction model (M1) as the base model, the heat accumulation effect of free convection at the model boundary is obtained using the solution results of the free convection model (M2) and the pure heat conduction model (M1) (i.e., the heat flux at the aforementioned model boundary), and this is taken as the free convection thermal effect of the target region. The isobaric boundary condition sets a constant pressure at the top boundary of the mathematical model (the upper surface of the mathematical model, usually referring to the Earth's surface) to represent a horizontal water level. Next, thermal boundary conditions, isobaric boundary conditions, and head boundary conditions are simultaneously configured in the mathematical model to form a hybrid convection model (M3) combining free convection and forced convection (i.e., the hybrid convection model (M3) is based on the free convection model (M2) with topographically driven forced convection superimposed). By solving the hybrid convection model (M3) and further using the free convection model (M2) as the base model, the heat accumulation effect of forced convection at the model boundary is obtained using the solution results of the hybrid convection model (M3) and the solution results of the free convection model (M2) (i.e., the heat accumulation effect of free convection at the aforementioned model boundary), and this is used as the forced convection thermal effect of the target area. The head boundary condition sets a specific head value at the top boundary of the mathematical model (the upper surface of the mathematical model, usually referring to the Earth's surface), whose value is approximately equal to the surface elevation, to simulate the actual slope of the water table. The initial temperature field is set according to the function relationship T = T0 + G*Z (G is the geothermal gradient, and Z is the stratum depth).

[0058] In the step of obtaining the heat accumulation effect of free convection at the model boundary as the free convection thermal effect of the target area, the same simulation parameters as those used in the numerical simulation of the geothermal geological mathematical model with thermal boundary conditions are employed. Numerical simulation is then performed using a geothermal geological mathematical model with both thermal and isobaric boundary conditions. This yields the heat flux at the model boundary under the combined effects of conduction and fluid heat transfer, independent of topographic relief. Based on this, and in conjunction with the heat flux of the geothermal geological mathematical model with thermal boundary conditions, the heat accumulation effect of free convection at the model boundary is calculated. Specifically, in this embodiment, the same simulation parameters as those used in the numerical simulation of the pure heat conduction model (M1) are used to perform numerical simulation using the free convection model (M2). The free convection model (M2) is then solved to obtain the temperature distribution pattern of the target area under the current simulation conditions. Based on this, the aforementioned temperature distribution pattern is analyzed to obtain the heat flux at the model boundary under the combined effects of conduction and fluid heat transfer, independent of topographic relief. Next, by subtracting the heat flux at the model boundary of the pure heat conduction model (M1) from the heat flux at the model boundary under the combined action of conduction and fluid heat transfer, which is independent of terrain undulation, the heat accumulation effect of free convection at the model boundary can be obtained.

[0059] In the step of obtaining the forced convection heat accumulation effect at the model boundary as the forced convection thermal effect of the target area, numerical simulation is performed using the same simulation parameters as a geothermal geological mathematical model with both thermal and isobaric boundary conditions. This yields the heat flux at the model boundary under the combined effects of conduction and fluid heat transfer, which is related to topographic relief. Based on this, the forced convection heat accumulation effect at the model boundary is calculated, in conjunction with the free convection heat accumulation effect at the model boundary. In this embodiment, numerical simulation is performed using a mixed convection model (M3) with the same simulation parameters as the free convection model (M2). The mixed convection model (M3) is then solved to obtain the temperature distribution pattern of the target area under the current simulation conditions. Based on this, the aforementioned temperature distribution pattern is analyzed to obtain the heat flux at the model boundary under the combined effects of conduction and fluid heat transfer, which is related to topographic relief. Next, by subtracting the heat flux at the model boundary of the free convection model (M2) from the heat flux at the model boundary under the combined action of conduction and fluid heat transfer, which is related to the topographic relief, the heat accumulation effect of forced convection at the model boundary can be obtained.

[0060] Next, after constructing the geothermal geological mathematical model, this invention further performs gridding on the constructed geothermal geological model to obtain realistic heat flux at the model boundaries, the heat accumulation effect of free convection at the model boundaries, and the heat accumulation effect of forced convection at the model boundaries. Specifically, this embodiment performs gridding on the constructed geothermal geological model, dividing the continuous physical region into discrete grids. Finite element grids are used to divide the model into many smaller domains, called elements (i.e., small, interconnected, and non-overlapping finite elements). Then, the above-mentioned set of equations is solved on these elements. These equations are approximately represented by a set of polynomial functions defined on each element to represent the required governing equations. As the grid is continuously refined, these elements become smaller and smaller, thus making the solution increasingly closer to the actual solution. Accordingly, the gridded geothermal geological model is used to obtain realistic heat flux at the model boundaries, the heat accumulation effect of free convection at the model boundaries, and the heat accumulation effect of forced convection at the model boundaries. For example, using a free triangular mesh with a mesh resolution of 0.1m to 150m, the Delaunay method (where no four points are concyclic and two adjacent triangles form the diagonal of a convex quadrilateral, and after exchanging them, the smallest of the six interior angles no longer increases) is used to complete the meshing of the constructed geothermal geological model.

[0061] In one specific embodiment of this application, the solution of the geothermal geological mathematical model with both thermal and isobaric boundary conditions, as well as the solution of the geothermal geological mathematical model with both thermal, isobaric, and hydraulic boundary conditions, are both performed using Comsol or Feflow multiphysics finite element simulation software.

[0062] Specifically, the local differential equations of each element in the gridded geothermal geological model are first converted into linear algebraic equations through numerical integration. This involves selecting finite polynomial functions, superimposing them, and then requiring the weighted integral of the results within the solution domain and at the boundary to satisfy the original equations. This yields a set of easily solvable linear algebraic equations. These equations are then combined into a large sparse matrix system for fluid-thermal coupling calculations. Since the physical properties of groundwater change with temperature, the matrix elements need to be dynamically updated based on the current temperature solution in each iteration. In short, this involves providing a reasonable initial temperature, velocity, and pressure field as the starting point for iteration. Starting from the initial temperature, velocity, and pressure fields, for each iteration, firstly, the viscosity, heat capacity, and density of the groundwater are calculated based on the current temperature solution. Then, based on the updated physical properties, the discretized equations for groundwater flow and heat transfer are recalculated and reassembled. Next, the updated equations are solved. Once new velocity, pressure, and temperature fields are obtained, they are fed back into the model to update the field variables throughout the domain. This process is repeated continuously in the nonlinear iterations until all relevant physical quantities reach the convergence criterion (e.g., the residual decreases to a certain threshold or the change in the solution is less than a certain threshold). For transient simulations, a time-stepping method is used, dividing the entire simulation period into a series of small time steps. The above solution steps need to be repeated at each time step until the solution converges at that time step. After completing the calculation for one time step, the solution of that time step is used as the initial value for the next time step, ensuring the continuity and stability of the time series. Accordingly, the goal is to solve geothermal geological mathematical models that simultaneously have thermal boundary conditions and isobaric boundary conditions, as well as geothermal geological mathematical models that simultaneously have thermal boundary conditions, isobaric boundary conditions, and hydraulic head boundary conditions.

[0063] Example 2

[0064] Based on the method for determining the convective heat effect of a karst geothermal system as described in Embodiment 1 above, this embodiment of the invention also provides a system for determining the convective heat effect of a karst geothermal system (hereinafter referred to as the "convective heat effect determination system").

[0065] Figure 2 This is a block diagram of the system for determining the convective heat effect of a karst geothermal system, according to an embodiment of this application. Figure 2As shown, the convective heat effect determination system in this embodiment of the invention includes: a parameter acquisition module 21, a model construction module 22, a first thermal effect acquisition module 23, and a second thermal effect acquisition module 24. Specifically, the parameter acquisition module 21 is implemented according to the method described in step S110 above, configured to measure the thermal property parameters of rock samples taken from the target area and convert them to a cylindrical coordinate system to form a rock thermal property column; the model construction module 22 is implemented according to the method described in step S120 above, configured to use the rock thermal property column to construct a geothermal geological model, and based on this, combined with the flow characteristics and heat transfer characteristics of groundwater in the target area, construct a geothermal geological mathematical model; the first thermal effect acquisition module 23 is implemented according to the method described in step S130 above, configured to only acquire thermal boundary values. The conditions are configured in the mathematical model to obtain the heat flux at the model boundary; the second thermal effect acquisition module 24 is implemented according to the method described in step S140 above, configured to simultaneously configure the thermal boundary conditions and the isobaric boundary conditions in the mathematical model, and combined with the heat flux, obtain the heat accumulation effect of free convection at the model boundary as the free convection thermal effect of the target area, and further simultaneously configure the thermal boundary conditions, the isobaric boundary conditions and the head boundary conditions in the mathematical model, and combined with the heat accumulation effect of free convection, obtain the heat accumulation effect of forced convection at the model boundary as the forced convection thermal effect of the target area.

[0066] This invention proposes a method and system for determining the convective heat effect of karst geothermal systems. The method first forms a rock thermal property column based on the thermal property parameters of rock samples taken from the target area. Then, a geothermal geological model is constructed based on the formed rock thermal property column, without considering the flow and heat transfer states of groundwater. On this basis, the flow and heat transfer characteristics of groundwater are then incorporated into the geothermal geological model to construct a geothermal geological mathematical model. Finally, for the geothermal geological mathematical model, thermal boundary conditions are configured only, thermal boundary conditions and isobaric boundary conditions are configured simultaneously, and thermal boundary conditions, isobaric boundary conditions, and hydraulic head boundary conditions are configured simultaneously. Ultimately, the heat accumulation effect of free convection at the model boundary is obtained as the free convection heat effect of the target area, and the heat accumulation effect of forced convection at the model boundary is obtained as the forced convection heat effect of the target area. This invention enables a quantitative study of the heat accumulation mechanism of karst geothermal systems, scientifically understanding the heat accumulation mechanism of geothermal systems, and providing a basis for determining the location of high-temperature, high-yield geothermal wells in karst geothermal reservoirs.

[0067] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

[0068] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the claims of the present invention.

[0069] Those skilled in the art will understand that the modules or steps of the present invention described above can be implemented using general-purpose computing devices. They can be centralized on a single computing device or distributed across a network of multiple computing devices. Optionally, they can be implemented using computer-executable program code, thereby storing them in a storage device for execution by a computing device, or fabricating them separately as individual integrated circuit modules, or fabricating multiple modules or steps as a single integrated circuit module. Thus, the present invention is not limited to any particular hardware and software combination.

[0070] While the embodiments disclosed in this invention are as described above, the content is merely for the purpose of facilitating understanding of the invention and is not intended to limit the invention. Any person skilled in the art to which this invention pertains may make any modifications and variations in form and detail of the implementation without departing from the spirit and scope disclosed herein; however, the scope of patent protection for this invention shall still be determined by the scope defined in the appended claims.

Claims

1. A method for determining the convective heat effect of a karst geothermal system, characterized in that, include: The thermal properties of rock samples taken from the target area are measured and converted to cylindrical coordinates to form a rock thermal property column. Using the aforementioned rock thermal property column, a geothermal geological model is constructed. Based on this, combined with the flow and heat transfer characteristics of groundwater in the target area, a geothermal geological mathematical model is constructed. By configuring only the thermal boundary conditions in the mathematical model, the heat flux at the model boundary is obtained; Simultaneously, the thermal boundary conditions and isobaric boundary conditions are configured in the mathematical model, and combined with the heat flux, the heat accumulation effect of free convection at the model boundary is obtained as the free convection heat effect of the target region. Furthermore, the thermal boundary conditions, isobaric boundary conditions, and head boundary conditions are configured in the mathematical model, and combined with the heat accumulation effect of free convection, the heat accumulation effect of forced convection at the model boundary is obtained as the forced convection heat effect of the target region.

2. The method according to claim 1, characterized in that, The step of constructing a geothermal geological model using the aforementioned rock thermal property column includes: Based on geophysical logging and borehole data, the target area is divided into several rock stratigraphic units. Then, based on the division results, the rock thermal properties are integrated with each rock stratigraphic unit to construct the geothermal geological model, which can be a two-dimensional or three-dimensional model.

3. The method according to claim 1 or 2, characterized in that, The method further includes: By treating the target area as an equivalent porous medium and combining the characteristics of the physical parameters of groundwater with temperature variation, the coupling relationship between the temperature field and the seepage field within the target area is determined, so as to obtain the flow characteristics and heat transfer characteristics of groundwater within the target area.

4. The method according to claim 3, characterized in that, The flow and heat transfer characteristics of groundwater within the target area are represented by the following expression: in, ρ represents the porosity of the equivalent porous medium. w The density of groundwater is represented by t, and time is represented by t. Let represent the gradient operator, q represent the groundwater velocity, K represent the permeability coefficient, and g represent the gravitational acceleration. The pressure gradient representing the direction of gravity. c represents the height gradient. w ρ represents the specific heat capacity of groundwater. ma c represents the density of porous media. ma This represents the specific heat capacity of the porous medium, where T represents temperature. λ represents the temperature gradient. w λ represents the thermal conductivity of groundwater. ma Q represents the thermal conductivity of porous media. r This indicates that the rock is radioactively generating heat.

5. The method according to claim 4, characterized in that, The variation characteristics of the physical parameters of the groundwater with temperature are represented by the following expression: c w (T)=12010.14-80.40T+0.31T 2 -5.38×10 -4 T 3 +3.63×10 -7 T 4 μ w (T)=0.004-2.108×10 -5 T+3.858×10 -8 T 2 -2.397×10 -11 T 3 r w (T)=1002.4-0.1905T-0.0025T 2 Where, μ w This indicates the dynamic viscosity of groundwater.

6. The method according to any one of claims 1 to 5, characterized in that, The steps for obtaining the heat flux at the model boundary include: Numerical simulation is performed using a geothermal geological mathematical model configured with the aforementioned thermal boundary conditions to obtain the geothermal gradient of the model under conduction heat transfer only. Then, the geothermal gradient is combined with the thermal conductivity of the model to calculate the heat flux at the model boundary.

7. The method according to claim 6, characterized in that, The step of obtaining the heat accumulation effect of free convection at the model boundary as the free convection heat effect of the target region includes: Numerical simulations are performed using the same simulation parameters as those used in the geothermal geological mathematical model configured with the aforementioned thermal boundary conditions. Numerical simulations are then performed using a geothermal geological mathematical model configured with both thermal and isobaric boundary conditions to obtain the heat flux at the model boundary under the combined effects of conduction and fluid heat transfer, independent of topographic relief. Based on this, and in conjunction with the heat flux of the geothermal geological mathematical model configured with the aforementioned thermal boundary conditions, the heat accumulation effect of free convection at the model boundary is calculated.

8. The method according to claim 7, characterized in that, The step of obtaining the forced convection heat accumulation effect at the model boundary as the forced convection heat effect of the target region includes: Using the same simulation parameters as the geothermal geological mathematical model that simultaneously configures the thermal boundary conditions and the isobaric boundary conditions, numerical simulation is performed using the geothermal geological mathematical model that simultaneously configures the thermal boundary conditions, the isobaric boundary conditions, and the hydraulic head boundary conditions. This yields the heat flux at the model boundary under the combined action of conduction and fluid heat transfer, which is related to topographic relief. Based on this, and in conjunction with the heat accumulation effect of free convection at the model boundary, the heat accumulation effect of forced convection at the model boundary is calculated.

9. The method according to any one of claims 1 to 8, characterized in that, After constructing the geothermal geological mathematical model, the method further includes: The constructed geothermal geological model is gridded to obtain the actual heat flux at the model boundary, the heat accumulation effect of free convection at the model boundary, and the heat accumulation effect of forced convection at the model boundary.

10. A system for determining the convective heat effect of a karst geothermal system, characterized in that, The system includes the following modules: The parameter acquisition module is used to measure the thermal physical parameters of rock samples taken from the target area and convert them to a cylindrical coordinate system to form a rock thermal physical parameter column. The model building module is used to construct a geothermal geological model using the rock thermal property column. Based on this, a geothermal geological mathematical model is constructed by combining the flow characteristics and heat transfer characteristics of groundwater in the target area. The first thermal effect acquisition module is used to configure only the thermal boundary conditions in the mathematical model to obtain the heat flux at the model boundary. The second thermal effect acquisition module is used to simultaneously configure the thermal boundary conditions and the isobaric boundary conditions in the mathematical model, and in combination with the heat flux, to acquire the heat accumulation effect of free convection at the model boundary as the free convection thermal effect of the target region. Furthermore, it simultaneously configures the thermal boundary conditions, the isobaric boundary conditions, and the head boundary conditions in the mathematical model, and in combination with the heat accumulation effect of free convection, to acquire the heat accumulation effect of forced convection at the model boundary as the forced convection thermal effect of the target region.

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