| Geothermal energy stored in the earth,especially Hot Dry Rock(HDR)geothermal energy,is a kind of renewable clean energy with extremely abundant reserves,which plays an irreplaceable role to address the future energy crisis.However,the utilization of HDR energy is difficult due to its extremely poor permeability,where engineering techniques are necessary to enhance the permeability.The traditional technique,called Enhanced Geothermal System(EGS),adopts the hydraulic fracture for the permeability enhancement of HDR,which relies too much on the geological conditions and fracture characteristics of the thermal reservoir.The normal quality of the reconstructed reservoir is unable to meet the limitation of commercialization since the fracture distribution cannot be controlled well.EGS is still on the learning curve,and the total installed capacity of EGS power generation is only 17.85Mwe all over the world by 2020.The mining industry forms a technical and equipment system suitable for deep mining,which safely mines mineral resources under different geological conditions.Therefore,Tang Chun'an et al.proposed the Enhanced Geothermal System based-excavation(EGS-E),which provides a new solution for the commercial exploitation of HDR energy.In his paper,the engineering framework and process flow were detailed,and associated issues on high-temperature rock damage,seepage,and heat transfer were studied.The main work and research results as follows:(1)On the basis of the conceptual model of the Enhanced Geothermal System basedexcavation,the engineering framework and process flow were detailed,and its engineering technical advantages in thermal storage transformation and thermal energy extraction were explained.The scientific research system of EGS-E was built in which the key scientific issues under the high-temperature and high-pressure environment were condensed,which laid the foundation for subsequent related research work.(2)By using experimental methods,the physical and mechanical properties of granites with different mineral grain distribution characteristics after different heating-cooling treatments were studied,as well as associated crack evolution.The uneven distribution of mineral grains(CV)has a significant impact on the physical and mechanical properties of granite samples under high-temperature treatment,resulting that earlier and more cracks,as well as more variation in the physical and mechanical properties in the specimen with higher CV.With the increase of the experimental temperature,the heating treatment damaged granite specimens irreversibly,leading to the variation in physical and mechanical properties,and the cooling treatment further enhances these damages and changes.The effect of the cooling treatment on the high-temperature granite is not obvious when the temperature is lower,and gradually increases with the increased temperature,in which its influence on the mechanical properties is higher than that of physical properties.Liquid nitrogen has a lower temperature and a higher convective heat transfer coefficient,so it has a better cold shock effect and stronger joint-making ability than water,which verifies the rationality of using the ultra-low temperature anhydrous liquid as a working fluid for enhancing geothermal reservoirs.For thermal reservoir with a temperature lower than 200?,the CV value of granite and the cooling fluid have almost no effect on the thermal storage enhancement;when the temperature reaches 400?,selecting the rock mass with high CV value or choosing liquid-nitrogen as the cooling fluid can get a better thermal reservoir enhancement.(3)By using RFPA-Thermal2D and COMSOL Multiphysics software,the thermal-solid coupling and thermal-fluid coupling models were built respectively.The temperature evolution and damage mechanism of the surrounding rock in a high-temperature tunnel were discussed,as well as the airflow temperature evolution before and after paving a thermal insulation layer.The results show that:The surrounding rock temperature was decreased unevenly under the cooling treatment by the lower temperature airflow,which induced thermal stress in the rock mass.This stress may damage to the rock mass and form damage units when exceeding the tensile strength of the rock mass.The damage units increase with increasing the ventilation time,and gradually coalesce and nucleate,eventually forming circular cracks at equal intervals in the tunnel rock mass and decreasing the tunnel stability.The thermal insulation layer weakens the thermal effect by the low-temperature airflow,delaying the time of rock damage and the initiation of cracks,reducing the damage cracks,and decreasing the tunnel airflow temperature,as well as extending the valid ventilation distance.The thermal insulation effect is closely related to factors such as the thermal insulation layer,ventilation conditions and surrounding rock temperature.The thermal insulation effect gradually increases with increasing of the thermal insulation layer thickness and the airflow speed and decreasing the thermal conductivity of the thermal insulation layer and the initial airflow temperature.The valid ventilation distance can characterize the cooling difficulty and insulation cost of hightemperature tunnels,whose formula can help geothermal engineers to calculate the valid ventilation distance conveniently and accurately under different surrounding rock temperatures and working conditions.(4)By using COMSOL Multiphysics software,a solid-thermal coupling and a fluid-solidthermal coupling model were built.The competitive influence of rock mass cooling shrinkage and fluid pressure on the evolution of crack apertures in the geothermal system was studied,and the influence of the thermal reservoir enhancement method on the efficiency of heat energy extraction was discussed.The results show that the influence of cooling shrinkage and fluid pressure on the crack aperture is in a trade-off relationship.The fluid pressure dominates the evolution of the crack aperture in the early stage of injection.The cooling shrinkage of the rock mass enhances as the injection time increases and became the main controlling factor of the crack aperture evolution in the later period.Increasing the cooling shrinkage of the bedrock and the fluid pressure in cracks both increase the total crack aperture.When the cooling shrinkage plays a leading role,the system injection capacity increases,while the system injection capacity decreases as the fluid pressure plays a dominant role.Using enhanced technology to generate random cracks in the thermal reservoir is the most effective way to improve the efficiency of heat energy extraction.EGS-E should choose the equal interval blasting method to generate uniformly distributed random cracks as much as possible to obtain the best heat extraction efficiency.The results have further enhanced the understanding of the crack aperture evolution and thermal energy extraction efficiency during the operation of the geothermal system,providing certain assistance to geothermal engineers in project site selection,thermal storage transformation and operating parameter optimization. |
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