Study On The Geochemical Characteristics Of Geothermal Water And Resources Assessment In The Qia Bu-qia Geothermal Area Of Gong He Basin In QingHai Province

Geothermal Energy has been a newly exploited and utilized energy resource for nearly a century, whose distribution is of a wide range and reserves rich. Based on detailed analysis of existing data and the results of previous studies, aiming at Qia Bu-qia geothermal area in Gong He Basin, Qing Hai province, this paper systematically studied the chemical characteristics and regional distribution of the underground geothermal water in Qia Bu-qia, focusing on the application of temperature scale geochemical method to estimate the temperature of thermal energy storage; at the same time, from the aspects of medical mineral use and corrosiveness, the water quality is evaluated. At last, using numerical simulation of groundwater flow, the safe yield of the Quaternary Pleistocene groundwater was evaluated, and the safe yield of the Tertiary groundwater was estimated by using the analogy method, to provide the basic information and scientific basis for the exploitation of geothermal water in this area. The main content of research and conclusions are as follows:(1) By the analysis of chemical composition of geothermal water through 21 sampling points of Qia Bu-qia area, the conclusions are: for the Quaternary groundwater, the major cation is Na⁺, and then Ca²⁺ and Mg²⁺; and the major anions are HCO₃^- and Cl^-, and then SO₄^2-; the major cation and anion of Tertiary groundwater are in accord with the Quaternary. The main water type in study area is Cl·HCO₃-Na, and also Cl·SO₄^-Na and HCO₃-Na·Mg·Ca.(2) The regional variation of TDS, water hardness, content of macro-component, hydrochemical type of Quaternary underwater are studied, the results show that: The distribution of Quaternary underwater of Qia Bu-qia area is obvious. The TDS gradually increases from north to south, the largest TDS is in south.; the hardness of the geothermal water is low, the water types are mainly soft water and weak hard water; Na⁺, Cl^- and HCO₃^-gradually increase from north to south, the Ca²⁺ and SO₄^2- are high in south and north region, and low in the middle region; the anionic division in this area is obvious, in the north is single sulfate-type water, in the middle are sulfate and bicarbonate compound water, in the south is multi-typed compound water. The cationic division is relatively complex. In general, in north and south is complex Na-Ca-Mg compound water, in the middle is Na-Mg compound water. From north to south, the chemical types of water varies as, HCO₃·Cl-Na→Cl-Na→Cl·SO₄ -Na→Cl·HCO₃·SO₄-Na→HCO₃·Cl-Na→HCO₃-Na→HCO₃·Cl - Na, The central region of the more complicated types of water chemistry. (3) The commonly used geothermometer and its application conditions are summerized, and the chemical analyzing data are simulated by PHREEQC. The saturation index of various types of minerals in the water are calculated, in the meanwhile, the method which using the SI-T graph to ascertain the equilibrium state is introduced. Taking spring water and geothermal wells for example, based on the analysis result of chemical composition, the water-rock balance is determined; using commonly used geothermomoter as quartz, chalcedony, Na/K, Na-K-Ca and K/Mg, the storage temperature of the geothermal water is estimated; based on the conditions of use and the actual situation of geothermal water resource in Qia Bu-qia area, the applicability of geothermometer and the reliability of estimation is disscussed, finally concluded that the storage temperature of Quaternary groundwater in Qia Bu-qia is between 40℃-75℃; and the temperature of Tertiary groundwater is about 70℃.(4) The water quality and geothermal water resource in research area are evaluated. According to the National Code of Geologic exploration standard of geothermal resources(GB11615—89) , the medical mineral function, carbon steel corrosion and drinking water quality are evaluated. The conclusion shows: the Quaternary geothermal water in south-east region reached the medical concertration, and its medical denominating projects are relatively more than north, the middle region relatively fewer; the geothermal water are mainly of minor corrosion and severe corrosion type;the major chemical composition over limits are NO₂^- and Fe, TDS,F^- and Cl^- are partly over limits. The fluoride and metasilicic acid content of Tertiary geothermal water reached the denominating concertration of mineral water, belongs to minor corrosion; the major composition over limits are Fe and F". Applying the numerical simulation of groundwater flow and the comparison of prediction modeling programs, it concluded that the safe yield of Quaternary Pleistocene groundwater is 43500m³/d, and the safe yield of the Tertiary groundwater is estimated to 6301.35m³/d by analogy method.