Well Cementation Composites Performance Research And Numerical Simulation Of Underground Heat Exchange Process For Mid-deep Geothermal Well

The heat energy of magma or hot dry rock more than two or three kilometers underground is often called mid-deep geothermal energy.Compared with shallow geothermal,there are the advantages of stability,high taste and economy.At present,it have become the focus of China's 13 th five-year plan for geothermal energy development and utilization.Casing type geothermal wells are used commonly in the mid-deep geothermal wells.It achieves the purpose of heat removal through heat exchange between circulating working medium flowing in and out of casing and well cementation materials.The thermal conductivity of well cementation materials is particularly important because of the characteristics of the depth of geothermal wells,which directly affects the efficiency of underground heat transfer.Meanwhile,the flow performance of cement slurry is particularly important,which is directly related to the quality and safety of cementing grouting.However,there is little research on this subject at present.Therefore,the research in this paper mainly focuses on the thermal conductivity of well cementation materials,the flow performance of cement slurry and the underground heat transfer for the mid-deep geothermal wells.Firstly,the cement-based materials are filled with iron powder of metal material,graphite of carbon material,silicon carbide,alumina(nano,micro)and other heatconducting particles to form well cementation composites.The influence of the types,particle sizes,grade ratio,water-cement ratio,curing time,curing environment and water reducer on the thermal conductivity of composites was studied,and prediction model of the thermal conductivity of composites is established.The results show that:in the single factor analysis: when filled with 10% graphite,thermal conductivity of composites increased from 1.038 w/(m · ?)to 1.644 w/(m · ?),increased by58.38%,far higher than that of mixed with the other fillers.At the same ratio,the thermal conductivity of the composites filled with nano-alumina is 5.12% higher than that of the composites filled with micron alumina.When mixed with various fillers,such as different fillers,different particle sizes or their collocation,these factors have different effects on improving thermal conductivity the well cementation composites.With the increase of water cement ratio and curing time,the thermal conductivity of well cementation materials decreases.The curing environment affect the overall improvement of thermal conductivity of well cementation materials,but the rules of different curing environments are nearly the same.The thermal conductivity of well cementation materials increased with suitable dosage of water reducer.Secondly,the experimental measuring device of cement slurry is designed and applied to the measurement experiment of rheological properties.The influence of the types,particle sizes,grade ratio,water-cement ratio and water reducer on the rheological properties of composites was studied,and prediction model of the rheological properties of composites is established.The results show that: in the single factor analysis: when 3% graphite is filled,the flow time of cement slurry have exceeded that of other fillers by 30%.When the fill ratio of micron and nanometer alumina is constant,the flow time of cement slurry increases with the content increases of nanometer alumina.When mixed with various fillers,such as different fillers,different particle sizes or their collocation,these factors have different effects on improving the rheological properties of cement slurry.With the increase of water cement ratio,the fluidity of cement slurry increases.There is an appropriate proportion of water reducer,and if the proportion exceeds,the improvement effect decreases significantly.The improvement is different because of the types of fillers.Finally,based on the well cementation composites formed above,the numerical simulation method is adopted to carry out the numerical simulation of underground heat transfer in a 2800 m mid-deep geothermal wells.The results show that: when the well cementation layer is not filled with materials,namely,the air is filled in well cementation layer,the heating efficiency of geothermal wells will decrease sharply.With the increase of thermal conductivity of well cementation composites the thermal efficiency of geothermal wells increases.However,when the thermal conductivity increases to 2.5w/(m·?),further increases the thermal conductivity of well cementation materials,the slope of the increase of the heat power of the whole geothermal well decreases significantly.The influence of thermal conductivity of well cementation materials on heat transfer of geothermal well system is greater than that of volume heat capacity.With the increase of geothermal well depth,the outlet water temperature increases continuously,and when the thermal conductivity of well cementation materials is higher,the average water temperature increases more.When well cementation materials with different thermal conductivity are used in different sections,the well cementation materials with high thermal conductivity is placed at the bottom of well cementation,the outlet water temperature is higher.