| Geothermal energy has broad prospects for utilization.Accelerating the development of geothermal resources,especially the hot dry rock,is of great significance for energy structure adjustment,energy-saving,emission-reduction and environmental improvement.Hot dry rock has the advantages of high temperature,wide distribution and large reserves,and is an important field for geothermal development in the future.In this study,a transient heat transfer model of formation-wellbore-fluid has been established based on the principle of geothermal exploitation from hot dry rocks via circulating water within a closed-loop horizontal well.The mechanism of heat transfer between the heat transmission fluid and the formation has been studied and the influences of formation parameters,wellbore parameters,injection parameters on the wellbore heat flux have been analyzed.The effects of thermal conductivity of the insulated tubing,the thermal conductivity of the cement sheath,the length of the horizontal section,the injection fluids and velocities and the circulating mode on the heat mining rate have been researched by simulation.Besides,the geothermal power generation abilities of different heat transmission fluids have been compared and the economic feasibilities of heat mining and power generation based on different conditions of geothermal formation using this technology have been studied.The results show that the main factors affecting the heat transfer between the formation and the heat transmission fluid include formation temperature,thermal conductivity of rock and wellbore,type of heat transmission fluid,injection rate and circulating mode.The employment of good heat-conducting cement and heat insulated tubing,the increase of the length of horizontal section,the utilization of negative circulating mode(injecting from the annulus and producing from the tubing)can effectively improve the heat mining rate.Compared with the continuous heat mining mode,the intermittent heat mining mode can improve the temperature of the heat transmission fluid but the overall heat mining capacity could be reduced.The feasibility of this technology for power generation shows that the heat transmission fluid with R134 a can obtain higher power generation and lower unit cost compared with water.Under the conditions of the vertical depth of the well 3500 m,the horizontal section 2000 m and the geothermal gradient 0.06 ?/m,the maxiam geothermal power can be 143.5 k W with water at the flow rate of 3 kg/s and the unit cost for power generation is 2.42 yuan/(k W·h).The maxiam net power can be increased to 340.1 k W with R134 a at the mass flow rate of 12 kg/s and the unit cost can be reduced to 1.18 yuan/(k W·h).Besides,considering the direct utilization of the geothermal energy,the unit cost for heat mining with water as the heat transmission fluid is0.158 yuan/(k W·h)at the same geothermal reservoir condition.The economic analysis of different conditions of geothermal formation shows that the lowest unit cost for heat mininig and power generation can be obtained at the length of horizontal section 1500 m and 2500 m respectively.The well cost and loan interest rate are two important factors affecting the unit cost of power generation and heat mining.Under the current price system,reducing the cost of well drilling and obtaining government support for lowering loan interest rates can greatly reduce the cost for the geothermal exploitation and can promote the development and application of geothermal technology. |
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