Research On The Hydrochemical Characteristics Of Geothermal Water And Its Formation In Bujiemu River Valley, Yangyi

This paper mainly studies hydrogeochemical characteristics of the geothermalwater and simulates the formation mechanism of geothermal water in Bujiemu Rivervalley, which locates at Yangyi Geothermal Field, Tibet, according to the analysis ofthe hydrogeological conditions and chemical data of geothermal water. meanwhile,the reservoir temperature both of the shallow and deep geothermal reservoir, scalingtendency and assessing CO2scale inhibition method are researched in this paper.Chemical Properties from the samples are analyzed, which show that thegeothermal water presents alkalescence with7.62-9.3of pH and the total dissolvedsolids (TDS) are low, and vary little ranging from1.4g/L to1.9g/l. The geothermalwater is characterized by a dominant HCO3-Na, behaved nature of external water. Thecomponents of geothermal water are derived from the rock dissolution. Assisted byCorrelation and scale coefficient Analysis, we know the Na+, Mg2+, HCO3-in thegeothermal water come from the dissolution of the silicate rocks, the dissolved biotiteand fluorite is supplied to the F-in the high temperature area and moderatetemperature area.It is concluded that the dissolution of CO2, plagioclase, biotite and calcite and theprecipitation of quartz, magnetite, hydrosialite (kaolinite, illite and fluorite) andfluorite happening during the geothermal water runoff in the shallow geothermalreservoir, the dissolution of CO2, calcite plagioclase, biotite and the precipitation ofquartz, magnetite, hydrosialite (kaolinite, illite and fluorite) taking place during thegeothermal water convection between shallow and deep geothermal reservoir. CO2acts as a activator,promoting the water-rock reaction.Cation geothermometer is not suitable for the temperature estimating because ofthe geothermal water mixed with the cold groundwater according to the Na-K-Mgtriangular diagram. The reservoir temperature in shallow is between180℃to240℃,while it is between about240℃in deep area using quartz geothermometer. Application of the multi-mineral balance method to calculate the temperature of thegeothermal water removes the cold groundwater, the reservoir temperature in shallowarea is between180to240℃, while it is between230℃and250℃in deep area.Carbonate scaling may occur in the study area, but then thesilicate scalingprobably not occur calculated by the index method and PHREEQC2.0programme.Dolomite and calcite are the possible scaling minerals, while the minerals precipitatedin the deep geothermal water are chlorite and calcite. Temperature, pH and CO2partial pressure are the main factors influence on the precipitation in the thermal water,injection CO2into the geothermal reservoir can prevent the scaling in the geothermalwater effectively, as it can also increase rock permeability.