CN112632875B - Enhanced geothermal system heat-taking working medium and determination method thereof - Google Patents
Enhanced geothermal system heat-taking working medium and determination method thereof Download PDFInfo
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- 238000000034 method Methods 0.000 title claims abstract description 18
- 239000011435 rock Substances 0.000 claims abstract description 44
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 claims abstract description 43
- 239000012530 fluid Substances 0.000 claims abstract description 27
- 229910052734 helium Inorganic materials 0.000 claims abstract description 26
- 239000001307 helium Substances 0.000 claims abstract description 26
- SWQJXJOGLNCZEY-UHFFFAOYSA-N helium atom Chemical compound [He] SWQJXJOGLNCZEY-UHFFFAOYSA-N 0.000 claims abstract description 26
- 238000004519 manufacturing process Methods 0.000 claims abstract description 24
- 229910002092 carbon dioxide Inorganic materials 0.000 claims abstract description 22
- 239000001569 carbon dioxide Substances 0.000 claims abstract description 21
- 239000011159 matrix material Substances 0.000 claims abstract description 18
- 230000000704 physical effect Effects 0.000 claims abstract description 6
- 238000004088 simulation Methods 0.000 claims abstract description 6
- 230000008878 coupling Effects 0.000 claims abstract description 4
- 238000010168 coupling process Methods 0.000 claims abstract description 4
- 238000005859 coupling reaction Methods 0.000 claims abstract description 4
- 238000002474 experimental method Methods 0.000 claims abstract description 4
- 238000003860 storage Methods 0.000 claims abstract description 3
- 238000000605 extraction Methods 0.000 claims description 9
- 238000012546 transfer Methods 0.000 claims description 5
- 230000002093 peripheral effect Effects 0.000 claims description 3
- 230000035699 permeability Effects 0.000 claims description 3
- 238000009413 insulation Methods 0.000 claims description 2
- 239000007787 solid Substances 0.000 claims description 2
- 230000000694 effects Effects 0.000 description 5
- 239000007789 gas Substances 0.000 description 4
- 238000004364 calculation method Methods 0.000 description 3
- 238000010586 diagram Methods 0.000 description 3
- 238000005338 heat storage Methods 0.000 description 3
- 230000008569 process Effects 0.000 description 2
- 238000011160 research Methods 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 238000009826 distribution Methods 0.000 description 1
- 238000003912 environmental pollution Methods 0.000 description 1
- 238000013213 extrapolation Methods 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 238000013178 mathematical model Methods 0.000 description 1
- 230000000877 morphologic effect Effects 0.000 description 1
- 238000010248 power generation Methods 0.000 description 1
- 238000005086 pumping Methods 0.000 description 1
- 230000007480 spreading Effects 0.000 description 1
- 238000003892 spreading Methods 0.000 description 1
- 238000010200 validation analysis Methods 0.000 description 1
- 238000012795 verification Methods 0.000 description 1
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F30/00—Computer-aided design [CAD]
- G06F30/20—Design optimisation, verification or simulation
- G06F30/28—Design optimisation, verification or simulation using fluid dynamics, e.g. using Navier-Stokes equations or computational fluid dynamics [CFD]
-
- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F2113/00—Details relating to the application field
- G06F2113/08—Fluids
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F2119/00—Details relating to the type or aim of the analysis or the optimisation
- G06F2119/08—Thermal analysis or thermal optimisation
Abstract
The invention provides a method for determining a heat-taking working medium of an enhanced geothermal system, which comprises the following steps: establishing a thermal-hydraulic coupling model of the hot dry rock based on physical property parameters of the deep hot dry rock thermal storage obtained by a rock physical experiment; constructing a cubic domain of a rock matrix, constructing a fracture channel with sine fluctuation morphology in the middle of the rock matrix by adopting a parametric surface method, determining the height of the fracture channel, and allowing fluid to flow into the fracture channel along the horizontal direction for heat exchange; simulation is carried out by using Commol Multiphysics to obtain different production temperatures of different working media under the same recharge flow and temperature. The invention determines that the heat-taking working medium with higher production temperature can be obtained according to the method as follows: supercritical carbon dioxide and helium are mixed according to the mass fraction of 9: 1 the mixed working medium obtained by mixing is a heat-taking working medium.
Description
Technical Field
The invention relates to a heat-taking working medium of an enhanced geothermal system for developing and utilizing geothermal energy of hot dry rocks, which is formed by mixing two working media, can realize higher production temperature and belongs to the field of application of geothermal energy of hot dry rocks.
Background
The utilization of geothermal energy has great significance for making up the uneven distribution of the traditional energy sources, relieving the problem of environmental pollution caused in the use process of the traditional energy sources and adjusting the energy source structure. Geothermal energy is a clean renewable energy source and has abundant reserves. The total available energy of geothermal heat is 5.4 x 10 estimated from the World Energy Council (WEC)27J, if only 1% of geothermal energy is utilized, the energy demand of 20000 years can be met. Compared with hydrothermal geothermal resources, the dry hot rock has low porosity, weak permeability, high temperature and rich reserves and is usually positioned3-10km underground is a clean energy which is currently researched and developed intensively in countries all over the world.
An enhanced geothermal system (EGS for short) is an effective means for developing and utilizing dry-hot rocks at present. A hot dry rock zone with a high temperature is selected and two or more parallel or near parallel wells are drilled in the zone as production wells and recharge wells. And constructing an artificial fracture network structure in a selected high-temperature rock region by means of hydraulic fracturing, wherein the artificial fracture network structure is called artificial heat storage. In the actual production process, heat exchange fluid is refilled into underground artificial heat storage through a refilling well for heat exchange, high-temperature fluid after heat exchange is pumped back to the ground surface through the production well and conveyed to a power plant for power generation and heat utilization, and low-temperature fluid after heat exchange is used for being refilled to the underground through the refilling well for heat exchange.
In order to efficiently extract heat in the dry-hot rock stratum, a working medium with excellent heat extraction property can be searched besides the improvement of a recharging mode. The enhanced geothermal system firstly adopts water as a heat-taking working medium. In 2000, enhanced geothermal systems with supercritical carbon dioxide as the heat-extracting working medium began to emerge. Compared with a water-based enhanced geothermal system, the supercritical carbon dioxide can reduce the pump power consumption required for pumping the supercritical carbon dioxide, and the interconnected fracture network of a plurality of secondary branches is easier to generate by using the supercritical carbon dioxide as a heat-taking working medium under the condition of relatively low fracturing pressure, so that a rock matrix generates a plurality of flow paths. Supercritical carbon dioxide has physical properties similar to the density and gas-like viscosity of liquids. Helium has small density, low viscosity and good heat conductivity, and is a working medium with very excellent heat exchange performance. Within the range known at present, helium is not found as a heat taking working medium of the enhanced geothermal system, and most of the current research is uniform working medium, and the excellent heat exchange characteristic of mixing two working media is not provided.
Reference documents:
[1]WANG J S,JIAO Y,LIU X L.Heat transfer and flow characteristics in a rectangular channel with small scale vortex generators.International journal of heat and mass transfer,2019,138:208–25.
[2]ZHANG L,JIANG P X,WANG Z,XU R N.Convective heat transfer of supercritical CO2 in a rock fracture for enhanced geothermal systems[J].Applied thermal engineering,2017,115:923-936.
disclosure of Invention
The invention aims to provide a method for determining a heat-taking working medium of an enhanced geothermal system and the determined heat-taking working medium. The invention mixes the supercritical carbon dioxide and the helium gas according to different mass fractions, provides a method for simulating the production temperature of different mixed working media and obtaining a new working medium, and the new working medium obtained by the invention can realize higher production temperature. The technical scheme is as follows:
a method for determining a heat-taking working medium of an enhanced geothermal system comprises the following steps:
1) establishing a thermal-hydraulic coupling model of the hot dry rock based on physical parameters of the deep hot dry rock thermal storage obtained by a rock physical experiment, wherein the physical parameters comprise temperature, permeability, density, thermal conductivity and specific heat;
2) constructing a cubic domain of a rock matrix, constructing a fracture channel with sine fluctuation morphology in the middle of the rock matrix by adopting a parametric surface method, determining the height of the fracture channel, and allowing fluid to flow into the fracture channel along the horizontal direction for heat exchange;
3) setting the density, specific heat, heat conductivity coefficient, initial temperature and initial pressure of the rock matrix; setting the temperature, the recharge flow and the outlet pressure of the recharge fluid; the upper wall surface and the lower wall surface of the cubic area adopt constant temperature boundary conditions; the peripheral wall surfaces adopt heat insulation boundary conditions, and the fluid adopts non-slip speed and non-step temperature boundaries on the upper wall surface and the lower wall surface of the fracture channel;
4) in the working range of underground heat extraction of the hot dry rock, the helium and the supercritical carbon dioxide are both in a supercritical state, an interpolation function changing along with the temperature and the pressure is generated according to physical parameters of a heat extraction working medium, and the density, the viscosity, the specific heat capacity and the heat conductivity coefficient of the supercritical carbon dioxide and the helium at different temperatures and pressures are respectively obtained under mixed working media with different helium mass fractions;
5) carrying out simulation by using a laminar flow module and a fluid and solid heat transfer module in Commol Multiphysics to obtain different production temperatures of different working media under the same recharge flow and temperature;
6) and determining that the heat-taking working medium with higher production temperature can be obtained.
The enhanced geothermal system heat-taking working medium determined by the method is characterized in that supercritical carbon dioxide and helium are mixed according to the mass fraction of 9: 1 the mixed working medium obtained by mixing is a heat-taking working medium.
The novel heat-taking working medium of the enhanced geothermal system provided by the invention has a better heat exchange effect, can obviously improve the production temperature of the system, improves the heat extraction capability of the working medium, and obviously improves the heat-taking effect when only supercritical carbon dioxide is used as the heat-taking working medium.
Drawings
FIG. 1 is a physical model of the present invention
FIG. 2 is a diagram of model validation
FIG. 3 is a trellis diagram
FIG. 4 shows production temperatures (10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% in the legend represents the mass fraction of helium gas)
FIG. 5 physical Property parameters (10%, 20%, 30% in the drawing represent the mass fraction of helium gas)
Detailed Description
The method is based on the physical parameters of the deep dry hot rock thermal reservoir obtained by rock physics experiments, including temperature, density, thermal conductivity, specific heat and the like, and a dry hot rock thermal-hydraulic coupling model is established. And determining the flowing and spreading rule of heat between the hot dry rock fracture and the heat taking fluid through simulation.
Firstly, a cube of 50mm multiplied by 50mm is constructed in Commol Multiphysics as a rock matrix, and a parametric surface method is adopted in the middle of the rock matrix to construct a fracture channel with sine wave morphology, wherein the height of the fracture channel is 0.2 mm. The fluid flows into the fracture channel along the horizontal direction to exchange heat. The specific morphological parameters of the fracture channel are as follows:
the physical model of the underground heat-extraction part of the enhanced geothermal system adopted by the invention is shown in figure 1.
The present invention makes the following assumptions: the reservoir rock mass is homogeneous, and the physical property of the reservoir rock mass is irrelevant to temperature and pressure; secondly, deformation of reservoir rock mass is not considered; thirdly, viscous dissipation in the fluid flow process is ignored; and fourthly, adopting a non-slip boundary condition for the fracture channel boundary. Based on the above assumptions, the mathematical model constructed by the invention is as follows:
continuity equation of the fluid:
where ρ isfFluid density, kg/m3(ii) a τ is time, s; u is the fluid velocity vector, m/s.
Momentum governing equation of fluid:
wherein, mufIs hydrodynamic viscosity, Pa · s; p is the fluid pressure, Pa.
Energy control equation of fluid:
wherein, TfIs the fluid temperature, K; lambda [ alpha ]fIs the thermal conductivity of the fluid, W/(m.K); c. CfJ/(kg. K) is the specific heat capacity of the fluid.
Energy equation in rock matrix:
wherein, TsIs the rock matrix temperature, K; lambda [ alpha ]sThe thermal conductivity of the rock matrix is W/(m.K); rhosDensity of rock matrix, kg/m3；csThe specific heat capacity of the rock matrix is J/(kg. K).
The initial parameters selected by the invention are as follows: the density of the rock matrix is 2700kg/m3The specific heat is 1250J/(kg.K), the heat conductivity coefficient is 3W/(m.K), the initial temperature of the rock matrix is 473.15K, and the initial pressure is 20 MPa; the temperature of the recharging fluid is 343.15K, the recharging flow rate is 0.35kg/h, and the outlet pressure is 20 MPa; calculating the upper wall surface and the lower wall surface of the domain by adopting a constant temperature boundary condition; the peripheral wall surface adopts an adiabatic boundary condition, and the fluid adopts a non-slip speed and non-step temperature boundary on the upper wall surface and the lower wall surface of the fracture channel.
In order to ensure the calculation accuracy, the invention divides five grids with different sizes and numbers, and performs Richardson extrapolation method[1]The verification is carried out, the number of the finally adopted grids is 190259, the grids of the crack part are locally encrypted, and a physical model meshing schematic diagram is shown in fig. 2.
In order to ensure the accuracy and reliability of the calculation result, the simulation result and the reference document of the invention[2]The experimental results in (1) are compared: when the recharging parameters and the physical parameters are consistent with the literature, the production temperature is 0.35kg/h and the maximum deviation of the experimental result is 0.1% when the fluid pressure is 8MPa and the inlet temperature is 340K, which shows that the calculation result of the research is accurate and reliable.
The mixed working medium selected in the invention is helium and carbon dioxide, and both are in a supercritical state. The mass fractions of helium gas in the mixed working medium are respectively 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90%, and the physical parameters of the 9 mixed working media are generated by REFPROP to generate interpolation functions changing with temperature and pressure, so as to respectively obtain the physical parameters of the mixed working medium such as density, viscosity, specific heat capacity, thermal conductivity coefficient and the like at different temperatures and pressures. The interpolation functions of the physical property parameters of the 9 mixed working media and the supercritical carbon dioxide and helium are respectively led into Comsol Multiphysics for simulation, so as to obtain different production temperatures of different working media under the same recharge flow and temperature, namely the temperature of the high-temperature working media extracted through the production well, as shown in FIG. 4.
Based on the enhanced geothermal system heat storage model and various mixed working media provided by the invention, the production temperature of helium under different mass fractions is researched to evaluate the heat extraction capability of different mixed working media, and the result shows that: when the mass fraction of helium is 10%, the production temperature of the mixed working medium is highest; the production temperature of pure helium is the lowest; in the mixed working medium, the production temperature is continuously reduced along with the increase of the helium proportion; when the mass fraction of the helium is 10% and 20%, the production temperature of the mixed working medium is higher than that of the supercritical carbon dioxide. As can be seen from fig. 5, in a certain temperature and pressure range, the specific heat capacity of helium is significantly higher than that of supercritical carbon dioxide, the specific heat capacity of the mixed working medium with helium mass fractions of 10% and 20% is lower than that of supercritical carbon dioxide, and when the helium mass fraction is 10%, the specific heat capacity of the mixed working medium is the lowest, so when the supercritical carbon dioxide and helium have a mass fraction of 9: 1, the highest production temperature can be realized, and a better heat exchange effect is achieved.
Therefore, the novel heat-taking working medium of the enhanced geothermal system provided by the invention has a better heat-exchanging effect, can obviously improve the production temperature of the system, improve the heat-extracting capacity of the working medium and obviously improve the heat-taking effect when only supercritical carbon dioxide is used as the heat-taking working medium.
Claims (2)
1. A method for determining a heat-taking working medium of an enhanced geothermal system comprises the following steps:
1) establishing a thermal-hydraulic coupling model of the hot dry rock based on physical parameters of the deep hot dry rock thermal storage obtained by a rock physical experiment, wherein the physical parameters comprise temperature, permeability, density, thermal conductivity and specific heat;
2) constructing a cubic domain of a rock matrix, constructing a fracture channel with sine fluctuation morphology in the middle of the rock matrix by adopting a parametric surface method, determining the height of the fracture channel, and allowing fluid to flow into the fracture channel along the horizontal direction for heat exchange;
3) setting the density, specific heat, heat conductivity coefficient, initial temperature and initial pressure of the rock matrix; setting the temperature, flow and outlet pressure of the recharging fluid; the upper wall surface and the lower wall surface of the cubic area adopt constant temperature boundary conditions; the peripheral wall surfaces adopt heat insulation boundary conditions, and the fluid adopts non-slip speed and non-step temperature boundaries on the upper wall surface and the lower wall surface of the fracture channel;
4) in the working range of underground heat extraction of the dry hot rock, helium and supercritical carbon dioxide are both in a supercritical state, an interpolation function changing along with temperature and pressure is generated according to physical property parameters of a heat extraction working medium, and the density, viscosity, specific heat capacity and heat conductivity coefficient of the supercritical carbon dioxide and the helium at different temperatures and pressures are respectively obtained under mixed working media with different helium mass fractions;
5) carrying out simulation by using a laminar flow module and a fluid and solid heat transfer module in Commol Multiphysics to obtain different production temperatures of different working media under the same recharge flow and temperature;
6) and determining that the heat-taking working medium with higher production temperature can be obtained.
2. The enhanced geothermal system heat extraction working medium determined by the method of claim 1, wherein supercritical carbon dioxide and helium are mixed according to the mass fraction of 9: 1 the mixed working medium obtained by mixing is a heat-taking working medium.
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| CN102384046A (en) * | 2011-06-24 | 2012-03-21 | 清华大学 | Energy conversion system used in intensified geothermal system with CO2 as working medium |
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| CN107100605A (en) * | 2017-04-21 | 2017-08-29 | 中国石油大学(北京) | A kind of method that dual horizontal well circulation supercritical carbon dioxide develops hot dry rock |
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| CN112065521A (en) * | 2020-09-16 | 2020-12-11 | 天津大学 | Based on CO2Mixed working medium supercharging heat absorption transcritical circulation hot dry rock geothermal power generation model |
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|---|---|---|---|---|
| CN102384046A (en) * | 2011-06-24 | 2012-03-21 | 清华大学 | Energy conversion system used in intensified geothermal system with CO2 as working medium |
| CN106870043A (en) * | 2017-04-18 | 2017-06-20 | 长沙紫宸科技开发有限公司 | The change system and method for carbon dioxide recycle generating are realized using geothermal energy |
| CN107100605A (en) * | 2017-04-21 | 2017-08-29 | 中国石油大学(北京) | A kind of method that dual horizontal well circulation supercritical carbon dioxide develops hot dry rock |
| CN107905778A (en) * | 2017-10-19 | 2018-04-13 | 中国石油大学(华东) | Supercritical CO2The enhanced geothermal system experimental provision of fluid fracturing and method |
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