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Seepage-heat transfer coupling process of low temperature return water injected into geothermal reservoir in carbonate rocks in Xian County, China
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Abstract: Fracture seepage and heat transfer in the geothermal reservoir of carbonate rocks after the reinjection of low temperature geothermal return water is a complex coupling process,which is also the frontier of geothermal production and reinjection research. Based on the research of cascade comprehensive development of geothermal resources in Beijing-Tianjin-Hebei (Xian County),the carbonate geothermal reservoir of Wumishan formation in the geothermal field in Xian County is investigated. With the development of the discrete fracture network model and the coupling model of seepage and heat transfer,the numerical solution of seepage field and temperature field with known fracture network is reached using the finite element software COMSOL,and the coupling process of seepage flow and heat in carbonate rocks is revealed. The results show that the distribution of temperature field of fractured rocks in geothermal reservoir of carbonate rocks has strong non-uniformity and anisotropy. The fracture network is interpenetrated,which constitutes the dominant channel of water conduction,and along which the fissure water moves rapidly. Under the influence of convective heat transfer and conductive heat transfer,one of the main factors to be considered in the study of thermal breakthrough is to make the cold front move forward rapidly. When the reinjection and production process continues for a long time and the temperature of the geothermal reservoir on the pumping side drops to a low level,the temperature of bedrocks is still relatively high and continues to supply heat to the fissure water,so that the temperature of the thermal reservoir on the pumping side will not decrease rapidly to the water temperature at the inlet of reinjection,but will gradually decrease after a long period of time,showing an obvious long tail effect. The distribution of fractures will affect the process of seepage and heat transfer in carbonate reservoirs,which should be considered in the study of fluid thermal coupling in carbonate reservoirs.
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Table 1. Computational parameter
Name of parameter Sign Numerical value Unit Fracture Porosity εfr 0.5 - Permeability kfr 1.0×10-8 m2 Porous media Density ρs 2 800 kg/m3 Specific heat capacity at constant pressure Cp, s 920 J/(kg·K) Thermal conductivity Ks 5 W/(m·K) Porosity εs 0.25 - Permeability ks 1.0×10-14 m2 Water Density ρf 1 000 kg/m3 Thermal conductivity Kf 0.68 W/(m·K) Specific heat capacity at constant pressure Cp, f 4 200 J/(kg·K) Fluid compressibility χf 0.000 1 1/Pa Dynamic viscosity μ 0.001 Pa·s Specific heat rate γ 1 - Boundary conditions and initial conditions Temperature of reinjection side T_in 30 ℃ Pressure of reinjection side P_in 20 MPa Pressure of mining side P_out 19.5 MPa Initial temperature of reservoir T_str 90 ℃ Initial pressure of reservoir P_str 19.5 MPa Boundary conditions The upper and lower boundaries are thermally insulated and fixed without flow 
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