Analysis Of Heat Extraction Performance Of Water And CO₂ In Supercritical Geothermal System

As a new type of green energy,compared with other clean energy sources,geothermal energy has the characteristics of huge reserves,and its development and utilization is not restricted by season or climate.Especially,supercritical geothermal energy has higher temperature and greater development potential compared with traditional geothermal energy.Based on the geological conditions of Larderello geothermal field,Italy,in this paper,we adopt the technical method combining"literature research and site data collection-theoretical analysis-numerical simulation application"to study the law of fluid and heat migration and the process of fluid phase transformation in the reservoir-wellbore coupled model during the process of supercritical geothermal development.And we aim to carry out energy efficiency analysis on a long-term power generation performance of the geothermal system.Subsequently,with the method of qualitative analysis and quantitative calculation,the influence of injection parameters（such as recharge temperature and pumping rate）over a lifespan of 100years was explored when water and CO₂were respectively used as working fluid.This study will provide a scientific basis for the project design of supercritical geothermal mining and a theoretical basis for setting injection conditions under different working fluid.In this paper,a coupled reservoir-wellbore coupled geothermal development model is established after conceptualizing the target supercritical reservoir.Through100-year operation simulation of geothermal system,this paper reveals the law of reservoir fluid and heat migration and phase transformation in the development process when water and CO₂are used as heat transfer working fluid,and evaluates the heat generation and power generation capacity of doublet-well geothermal system from multiple evaluation aspects.It is found that in reservoir,fluid mainly exist in the form of steam phase.When water is used as the heat transfer working fluid,the injected low-temperature fluid will cause the in-situ steam to liquefy gradually.As migrating to the production well,the liquefied fluid will absorb heat which can convert it back into steam.In the process of phase transition,the fluid absorbs heat in the flow path to make up for the enthalpy difference between two phases（steam and liquid）,thus the temperature is basically unchanged.As the two-phase region expands,the steam phase after phase transition does not have enough space to rise temperature,as a result of which,the temperature of flow generation will drop sharply in several years.At the initial stage of production,the produced fluid is all single-phase gas.At this time,the flow production temperature will decrease slightly due to bottomhole liquefaction,but the outlet temperature is relatively stable on the whole.With the outlet fluid transforms into gas-liquid coexistence,the outlet temperature will also drop sharply.At 100^thyear,the liquid phase accounts for 86.4%of the production fluid,and the flow production temperature will drop to 308.4 ^oC.The heat extraction efficiency is about 13.87 MW,while he power generation efficiency is 2.2MW,and the power generation potential of a single well is 2.6MW.The simulation results reveals supercritical geothermal system has great development potential compared with traditional geothermal energy.When CO₂ is used as geofluid,the fluid is always in supercritical phase without phase change under the condition of temperature and pressure of our model.When the reservoir temperature is 400 ^oC and the reservoir pressure is approximately 17.6 MPa,under the constant mass flow rate of 13.89 kg/s,the mobility（which is inversely proportional to kinematic viscosity of fluid）of CO₂is about 1.35 times that of water.Besides,the outlet temperature is higher and more stable than that of water system.After 100 years,the production temperature is 340.1 ^oC,which is 10.3%higher than water.The heat extraction efficiency is 5.63 MW and the power generation efficiency is 1.14 MW,which are 41%and 52%of the water-based geothermal system,respectively.The power generation potential per well is about 1.06 MW,which is about 41%of the water system.The change of recharge temperature has a great effect on the power generation efficiency of both geothermal systems,but has little effect on the flow generation temperature.However,it affects the mobility of water more than its effects to CO₂.The change of pumping rate has greater influence on the heat extraction efficiency and power generation capacity of CO₂geothermal system compared with water,but has little influence on out flow temperature.On the contrary,for the water system,it has a great influence on the temperature of flow production,but has little influence on the heat extraction efficiency and power generation capacity of the system.Therefore,for the supercritical geothermal system with water as the geofluid,the influence of impedance and out flow temperature should be considered respectively when selecting injection temperature and flow rate.For the geothermal system with CO₂as working geofluid,the injection temperature can be reduced as much as possible and the production flow can be increased to improve the heat extraction and power generation performance of the geothermal system.