Coupled Reservoir-Wellbore Study Of Flow And Heat Transfer Of Geothermal Reservoir

The extraction and reinjection of geothermal water is the traditional way of geothermal direct use,and it is also the most efficient way.The numerical simulation of the whole process of geothermal production and reinjection is of great significance to the dynamic productivity characteristics of geothermal well and resources evaluation,the results can provide a theoretical guidance for the geothermal utilization.Most previous numerical simulations did not consider the heat transfer process of the geothermal fluid from the reservoir to the wellhead and then reinjected,the geothermal well was considered as the source/sink.However,because the thickness of water intake section of geothermal wells is usually several meters to hundreds of meters,the different water intake locations affect the flow ratio at different positions of the aquifer and the velocity distribution at the vertical direction,thus affecting the water intake temperature.The coupled heat transfer of geothermal reservoir-wellbore includes the coupling of flow and heat transfer between the water intake section and the surrounding aquifer,and the heat transfer between the closed section at the upper part of geothermal wellbore and formation,which is a complex heat and mass transfer process.Due to the difficulty in arranging measuring points in the water intake section of wellbore and surrounding aquifer in practical applications,it is hard to obtain the required parameter to verify the numerical model.In contrast,lab-scale simulation experiments have the advantages of easy layout of measuring points,convenient data acquisition,and controllable parameters,which can provide experimental verification for numerical models.In this paper,a lab-scale simulated experimental system is designed and established,and the simulated formation with high temperature gradient can be quickly achieved through layered heating and boundary dynamic thermal compensation.Because it is difficult to conduct a lab-scale experiment of different water intake positions in the aquifer,this paper firstly conducts a theoretical study on the coupled flow of geothermal fluid and reservoir during geothermal production and reinjection,the analytical solution of pressure distribution along the well,the pressure distribution and seepage velocity in the formation are obtained,which agree well with the previous results.Based on the established simulated formation,the experiments of fluid flow rate,fluid temperature and temperature gradient on the temperature profile of reservoir caprocks during geothermal production and injection were carried out,and the temperature distribution along the wellbore and outlet water temperature under different conditions are obtained,which are in good agreement with the calculated results.The conclusions are as follows:(1)During geothermal production,the temperature of the reservoir caprock near the wellbore gradually increases.The greater the flow rate and temperature gradient,the higher the temperature of the reservoir caprock,and the flow rate has a greater impact on the upper part of the reservoir caprock; when the flow rate is 45 m L /min and 18 m L /min,the difference of the average temperature of the reservoir caprock along the wellbore is 1.7℃;when the temperature gradients are 26°C/m and 21°C/m,respectively,the difference in average temperature along the wellbore is 2.5°C.(2)During geothermal reinjection,the surrounding caprock of the wellbore gradually decreases.When the flow rate is 45 m L /min and 18 m L /min,the difference of the average temperature of the reservoir caprock along the wellbore is0.6℃;when the reinjected temperature is 15℃ and 20℃,the difference in average temperature along the wellbore is 3℃;when the temperature gradient is 26℃/m and21℃/m,the difference in average temperature along the wellbore is 2.8℃.(3)When the flow rate is 45 ml/min,25 ml/min and 18 ml/min during production,the corresponding maximum temperature drop of the fluid along the wellbore is 4.2°C,6.8°C and 8.8°C,respectively,and the effect of the flow rate on the temperature of the upper caprock is greater.The above conclusions can verify the coupled geothermal reservoir-wellbore simulation.