Study On Heat Transfer Characteristics And Thermal Energy Storage Performance Of Brackish Aquifers

Because of the brine and existence of clay minerals in brackish aquifer,permeability characteristics and heat transfer process in brackish aquifers are significantly different from those in fresh aquifers,especially when the salinity and temperature of injection fluid are different from those of the reservoir,which could result in a great change in permeability.In this paper,based on the brine quality and the hydrogeological characteristics of brackish aquifer in Tianjin coastal area,heat transfer characteristics in brackish aquifer with different injection parameters are calculated,also the influences of these parameters on thermal energy storage efficiency of brackish aquifer are analyzed.The hydraulic conductivity used in this simulation was obtained in the previous experimental test.Based on the theory of heat and mass transfer in porous media,a coupled mathematical model of seepage,heat transfer and mass transfer is established in this paper.In the calculation,the change of fluid physical parameters(density and viscosity)and hydraulic conductivity with temperature and salinity are considered.The model is verified by comparing the simulation results with the field experiment data and the actual engineering data of the literature.In different injection salinity,injection temperature,flow rate and well spacing conditions,the pumping water temperature and the underground temperature distribution in the brackish aquifer are calculated using COMSOL Multiphysics software.Thermal radius is obtained and the effects of injection parameters and well spacing on the characteristics of heat transfer and thermal energy storage performance are analyzed.The results show that as injection salinity decreases,velocity of heat migration in the reservoir slows down,the change of pumping water temperature reduces,and the influence area of the injection decreases,which is helpful to improve the thermal energy storage efficiency.As the injection salinity reduces from 5 g/L to 0 g/L,the cold storage efficiency increases from 22.06% to 25.09% and the heat storage efficiency increases from 33.09% to 35.51%.The influence of injection temperature on heat transfer is depended on the comprehensive affection of the temperature difference between the injection fluid and the brackish aquifer and the change of the permeability of the reservoir.The cold storage efficiency with 7 °C cold water injection is larger than that of 10 °C,as the injection water temperature is 60 °C,the heat storage efficiency is the biggest.The more injection flow rate and the larger change in the pumping water temperature and the underground temperature,which could speed the heat migration and lead to the increase of heat loss,then reduce the thermal energy storage efficiency.Enlarging well space delays the occurrence of heat penetration and reduces the change of pumping temperature,which effectively improves the thermal energy storage efficiency of the system.Through the simulation and prediction of long-term operation of the system,it is indicated that the unbalance of injected cold and heat reduces the thermal energy storage efficiency,and causes the reservoir temperature increasing or decreasing.Therefore,it is important to make an appropriate design in the injection parameters(temperature,flow rate and salinity)for improving the thermal energy storage efficiency and the stability of the system.