Genesis And Numerical Simulation Of Geothermal Resources In Shangcai County

Geothermal resources are rich in Henan Province and mainly characterized by middle and low temperature sedimentary basin type.There is few in depth studies on the genetic mechanism of geothermal resources in middle and low temperature sedimentary basins,over-exploitation results in waste of geothermal resources.Therefore,the geothermal field in Shangcai County,a typical medium-low temperature geothermal system in Henan Province,is selected as the study area to analyze the formation process and evolution of geothermal water system,which provides theoretical guidance and reference for the rational development and utilization of medium-low temperature geothermal system geothermal water.Taking Shangcai County as the study area,geophysical exploration and hydrogeochemical methods are comprehensively applied to analyze the geothermal field,geothermal fluid chemical characteristics and their genesis in this study.A coupling model of hydrodynamic field and temperature field of geothermal water is established by using FEFLOW software.The variation of hydrodynamic field and temperature field of geothermal water under different mining conditions is discussed.The main results are as follows:The average geothermal flow value in study area is 55m W/m²,the geothermal gradient is between 2.2?3.6?/100 m.The geothermal field of deep thermal reservoir is obviously affected by basement structure and faults.The geothermal gradient is higher at the boundary of structural units and the intersection of several faults,such as3.6?/100 m in Yangji Town;the geothermal gradient in other sections are smaller than 3.5?/100m.The geothermal water of Neogene and Paleogene in the study area has similar hydrochemical types(mainly HCO₃-Cl-Na).Geothermal water is characterized by higher content of Na⁺and HCO₃^-,lower Ca²⁺and Mg²⁺,and larger fluctuation of Cl^-.As a whole,the contents of TDS,Na⁺,SO₄^2-and Cl^-increase with the depth,of which Na⁺increases obviously;however,Ca²⁺and Mg²⁺decrease with the depth.According to the analysis of hydrogeochemical process,Na⁺and Cl^-come from the dissolution of rock salt,Ca²⁺and Mg²⁺come from the dissolution of calcite and dolomite,and SO₄^2-is from the dissolution of gypsum.The wellhead effluent temperature in study area is 41?57?,and the thermal storage temperature is estimated to be 40?60?by K-Mg temperature scale method.According to the analysis of mineral saturation index,there is obvious mixing effect between geothermal water and shallow cold water.The mixing ratio of cold water calculated by mixing ratio method is between 0.51 and 0.7.The?D and?¹⁸O of geothermal water are-70‰and-9.6‰,which are distributed around the global and national atmospheric precipitation lines,indicating that geothermal water comes from atmospheric precipitation.According to the elevation effect of?D and?¹⁸O,it is inferred that the source is from the atmospheric precipitation in western chaya mountain.The depth of geothermal water circulation ranges 1969?2963 m.According to tritium analysis,the geothermal water in the study area is"ancient water"supplied by infiltration before 1952,and ¹⁴C dating analysis shows that the geothermal water age in Shangcai is 34.92?39.22ka.Based on Shangcai geothermal geological conceptual model,the corresponding mathematical model is established,the three-dimensional numerical model of geothermal water is established to discuss the changes of the whole hydrodynamic field and temperature field of single well and model study area under the schemes of current exploitation,increasing exploitation amount,increasing intermission period and adding recharge wells.The water level and water temperature decrease faster with the increasing exploitation amount of single well;Given that there is intermission period,the water level recovers greatly during intermission period.For water temperature,although the temperature has rebounded,but the rising range is relatively low.The distance between the recharge well and the pumping well(400m?800m)not only affects the water level of the recharge well but also affects the water level of the pumping well.Well spacing has no effect on temperature.Different mining conditions have direct and great influence on the change of hydrodynamic field.The amount of groundwater exploitation has a great influence on the whole groundwater level but relatively small influence on the temperature field in the study area.