Numerical Investigation on the Influence of Areal Flow on EGS Thermal Exploitation Based on the 3-D T-H Single Fracture Model
Abstract
:1. Introduction
2. TH Coupled Single Fracture Model
2.1. The Governing Equation
2.1.1. Reservoir Flow Field Governing Equation
2.1.2. Reservoir Temperature Field Governing Equation
2.2. Model Verification
2.3. Model Establishment of Enhanced Geothermal System
2.4. Initial and Boundary Conditions
2.5. Thermal Recovery Evaluation System
- (1)
- Operating lifetime of EGS: the total operating time of the system from the initial operating temperature of the thermal fluid in the production well in EGS to below 150 °C.
- (2)
- Heat production rate of EGS:
- (3)
- Overall thermal recovery rate R(t) refers to the ratio of the heat energy extracted after the operating time of the system t to the total recoverable heat energy in the reservoir:
- (4)
- The local thermal recovery rate RL refers to the ratio of the thermal energy extracted from the local location of the reservoir to the total recoverable energy after the system operation time t:
- (5)
- Net power generation:
3. Influence of Regional Flow on EGS Thermal Mining Process
3.1. Influence of Regional Flow on EGS Fluid Field
3.2. Influence of Regional Flow on EGS Temperature Field
3.3. Influence of Well Spacing on Thermal Performance of EGS
3.4. Effect of the Flow Rate on Output Temperature
3.5. Influence of Areal Flow on EGS Thermal Energy Extraction
4. Conclusions
- (1)
- The existence of regional flow plays an important role in the flow behavior (size and direction) of fluids in EGS. From the numerical results, it can be found that there are two stagnation points regardless of the value of β. If the centers of the two stagnation points are connected into a straight line, the line rotates clockwise relative to the center of the system;
- (2)
- The ratio of the amount of fluid flowing into the production well from the injection well to the total fluid volume in the production well gradually decreases as the value of β increases;
- (3)
- After 50 years of EGS operation, with decreasing the β value, the output temperature and the service lifetime of EGS decreases, but the heat recovery rate increase; with increasing the β value, the output temperature and the service lifetime of EGS increase, but the heat recovery rate decrease. So that the reasonable choice of β value has a significant impact on the thermal recovery performance of EGS;
- (4)
- When only considering the influence of well spacing on EGS, with increasing the well spacing, the output temperature increases. But when the well spacing increases to a certain value, the well spacing will not affect the temperature of the production well. The study found that the effect of well spacing on the thermal recovery performance of EGS is magnified under regional flow;
- (5)
- When only the influence of the initial injection flow rate is considered, when β ≠ 0°, the larger the initial injection flow rate, the shorter the heat absorption time of the fluid, and the lower the production well temperature. The study found that the effect of initial injection flow on EGS thermal recovery is magnified under regional flow.
Author Contributions
Funding
Conflicts of Interest
Nomenclature
∇T | gradient operator |
cf | heat capacity of fracture water (J/kg·°C) |
Cp | heat capacity of the working fluid (J/(kg·K)) |
cp.fr | Specific heat capacity of the fracture (J/kg/K) |
cp.p | specific heat capacity of porous media |
cr | rock heat capacity (J/kg·°C) |
df | fracture aperture (m) |
keff | effective thermal conductivity |
kfr | thermal conductivity of fracture (W/m/K) |
kp | thermal conductivity of porous media (W/m/K) |
kr | rock heat conductivity (W/m·°C) |
L | fracture length (m) |
p | pressure (Pa) |
Qf,r | heat transfer between the porous media and fractures |
Qm | the mass transfer between the porous media and fractures |
R | ratio of the heat energy extracted |
RL | ratio of the heat energy extracted |
t | time (s) |
T | temperature in the porous media (K) |
T0 | minimum conversion temperature °C, the article assumes that it can reach 293.15 K (20 °C) |
Tc | the temperature in degrees Celsius (°C) |
Tf0 | temperature of fracture water (°C) |
Tin | initial fluid injection temperature |
Tr | initial rock temperature |
Tr0 | initial temperature of rock mass (°C) |
u | Darcy velocity (m/s) |
uf | Darcy velocity in the fracture (m/s) |
V | reservoir volume, m3 |
W | heat production rate of EGS |
We | net power generation |
Γ | Specific heat ratio of fluid |
δ | fracture aperture (m) |
E | porosity of the reservoir |
εf | fracture porosity |
εp | reservoir porosity |
H | dynamic viscosity (Pa·s) |
θfr | volume fraction of fracture boundary material |
θp | volume fraction of the porous medium |
κ | reservoir permeability (m2) |
κf | fracture permeability (m2) |
μ | water viscosity (Pa·s) |
ξ | conversion coefficient between thermal energy and electrical energy, the article takes 0.45 |
P | water density (kg/m3) |
ρf | density of fracture water (Kg/m3) |
ρfr | fracture density (kg/m3) |
ρr | rock density (Kg/m3) |
(ρcp)eff | effective volumetric capacity |
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Simulation Software | Development Agency | Computational Method | Software Feature |
---|---|---|---|
COMSOL | COMSOL Co. Ltd. Stockholm, Sweden | FEM | Self-defining partial differential equation, automatic solution, automatic division of the grid, multiple physical problems can bearbitrarily coupled |
FEHM | Operated by Los Alamos National Security, LLC for the U.S. Dept. of Energy’s NNSA | FEM | 3-dimensional complex geometries with unstructured grids saturated and unsaturated media simulation of production from gas hydrate reservoirs simulation of geothermal reservoirs non-isothermal, multi-phase flow of gas, water, oil non-isothermal, multi-phase flow of air, water non-isothermal, multi-phase flow of CO2, water and so on. |
ECLIPSE | Schlumberger, Central and Eastern Europe: Hannover, Germany | FVM | For all types of reservoirs and degrees of complexity, structure, geology, fluids, and development schemes. ECLIPSE is a fully implicit, three-phase, 3D, general purpose black-oil simulator that includes several advanced and unique features. |
Fluent | FLUENT Co. Ltd. Lebanon, NH, USA | FVM | Dynamic/deformed mesh technology mainlysolves the boundary motion, and the simulation fluid flow and heat transfer with high accuracy |
Parameters | Symbols | Units | Values |
---|---|---|---|
Initial temperature of rock mass | Tr0 | °C | 180 |
Temperature of fracture water | Tf0 | °C | 40 |
Rock density | ρr | Kg/m3 | 2820 |
Rock heat capacity | cr | J/kg·°C | 1170 |
Rock heat conductivity | kr | W/m·°C | 2.80 |
Density of fracture water | ρf | Kg/m3 | 1000 |
Heat capacity of fracture water | cf | J/kg·°C | 4200 |
Fracture flow velocity | uf | cm/s | 1 |
Fracture aperture | δ | m | 0.01 |
Fracture length | L | m | 100 |
Parameters | Symbols | Units | Values |
---|---|---|---|
Porous media permeability | к | m2 | 1.0 × 10−12 |
Porous media porosity | εp | 1 | 0.01 |
Porous media density | ρp | kg/m3 | 2700 |
Constant pressure heat capacity of porous media | cp.p | J/(kg·°C) | 1000 |
Thermal conductivity of porous media | k | W/(m·°C) | 3 |
Specific heat ratio of porous media | γp | 1 | 1 |
Fracture porosity | εfr | 1 | 0.5 |
Fracture permeability | кp | m2 | 1.0 × 10−10 |
Fracture density | ρfr | kg/m3 | 1650 |
Specific heat capacity of the fracture | cfr | J/(kg·°C) | 1000 |
Thermal conductivity of fracture | kfr | W/(m·°C) | 4 |
Fracture Width | dfr | m | 0.002 |
Specific heat ratio of fracture | γfr | 1 | 1 |
Constant pressure heat capacity of fluid | cp | J/(kg·°C) | 4200 |
Thermal conductivity of fluid | kp | W/(m·°C) | 0 |
Specific heat ratio of fluid | γ | 1 | 1 |
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Yao, C.; Shao, Y.; Yang, J. Numerical Investigation on the Influence of Areal Flow on EGS Thermal Exploitation Based on the 3-D T-H Single Fracture Model. Energies 2018, 11, 3026. https://doi.org/10.3390/en11113026
Yao C, Shao Y, Yang J. Numerical Investigation on the Influence of Areal Flow on EGS Thermal Exploitation Based on the 3-D T-H Single Fracture Model. Energies. 2018; 11(11):3026. https://doi.org/10.3390/en11113026
Chicago/Turabian StyleYao, Chi, Yulong Shao, and Jianhua Yang. 2018. "Numerical Investigation on the Influence of Areal Flow on EGS Thermal Exploitation Based on the 3-D T-H Single Fracture Model" Energies 11, no. 11: 3026. https://doi.org/10.3390/en11113026