Study On The Key Factors Of Geothermometers And Their Application

Geothermal energy is a kind of new green energy,and with the awareness of environmental protection and the shortage of energy increasing in recent years,people paid more attentions on the exploitation and utilization of geothermal resources.At the same time,the huge potentiality of geothermal energy makes the utilization of geothermal energy have a very large prospect.According to the occurrence and depth,geothermal resources can be divided into shallow geothermal energy,geothermal fluid and hot dry rock.At present,the hydrothermal geothermal resources（geothermal fluids）are the most widely used in China,and based on the reservoir temperatures,the hydrothermal geothermal systems are further divided into high-temperature geothermal systems（>150℃）and medium-temperature geothermal systems（150-90℃）.Also,the high temperature geothermal fluids can be widely used for geothermal power generating,drying and heating in the industrial field,while the low-temperature geothermal fluid（less than 90℃）is mainly used directly because of its suitable temperature,high quality water,and it contains a variety of trace elements and minerals,which benefits human body.Therefore,the low temperature fluids can always be used for heating houses,medical treatment,entertainments,planting and breeding,and so on.Before evaluating geothermal resources in hydrothermal geothermal systems,estimating the temperature of reservoirs is very important.Compared to gas geothermometers and isotope geothermometers.The classical geothermometers have great advantages.Therefore,in this study,to solve the problems existing in different geothermometers,the study of geothermometers will be further proceeded,and the key factors influencing the accuracy of geothermometers and their applicable conditions will be discussed.The main research contents of the thesis include:1.The hydrochemical data of geothermal fluids collected from geothermal wells around the world are collected,and the difference of water-rock interaction between different temperature crystalline reservoirs is analyzed,including the influence on potassium-related geothermometers.The estimates of potassium-related geothermometers are compared with the reservoir temperatures（including the measured geothermal well temperature or the estimated temperature by SiO₂ geothermometers）,aiming to analyse the applicability of different geothermometers with various reservoir temperatures overall.Then,the water samples with more accurate reservoir temperature were simulated by the software（WATCH）to obtain the activities of different hydrochemical components in reservoirs.The results show that:the reservoirs can be divided into high temperature heat reservoirs and medium-low temperature reservoirs with the boundary of about 200℃,and the situation of water-rock interaction is different,which will influence the accuracy of potassium-related geothermometers to some extent.K-feldspar and muscovite always control the concentration of K⁺and Al³⁺in geothermal fluid of mid-low temperature reservoirs,yet with reservoir temperature increasing,the Na-K ion-exchanging equilibrium controlled by K-feldspar and albite is easy to be achieved.Different with Na-K equilibrium,it will be easy that K-Mg reach the equilibrium in mid-low temperature reservoirs,even at lower temperature（<100℃）.However,in the high temperature reservoirs,K-Mg equilibrium in the geothermal fluids may be disturbed by many factors.There are two possible reasons:1)the equilibrium between K-feldspar and muscovite is replaced by Na-K ion-exchanging equilibrium between K-feldspar and albite;2)The concentrations of Mg in the high temperature fluids are so low that the K-Mg ratio will be influenced easily.The accuracy of Na-K-Ca geothermometer is related to the Na-K equilibrium,additionally,the Na-Ca ratio will not be controlled by the related minerals instead of being controlled by the Na-K and K-Ca equilibrium.When determining the equilibrium among Na-K-Mg in geothermal fluids,for the geothermal fluids derived from high-temperature reservoirs,The equilibrium line in the original Na-K-Mg triangular diagram can still be used to determine whether the geothermal fluid reaches the equilibrium via water-rock interacting in reservoirs,while the equilibrium relationship between SiO₂ and K-Mg can be better used to estimate the temperature of mid-low temperature reservoirs.2.In the case of Batang geothermal field and Xiangcheng geothermal field,this section studies the key factors influencing the accuracy of Na-K-Ca geothermometers,and further analyzes tthe difference for applying geothermometers in bicarbonate geothermal fluids with high TDS（total dissoloved solid）.Due to the absence of magmatic degassing,the geothermal water in the Batang geothermal field and Xiangcheng geothermal field is dominated by the high mineralized bicarbonate geothermal fluids.Similar to the geothermal system with magmatic heat source,the reservoir temperature is still the key factor that influencing the extents of water-mineral interaction.In the Batang geothermal field and Xiangcheng geothermal field,the estimates of the K-Mg geothermometer and chalcedony geothermometer are proven to be the better choices,and the temperature is about 180℃.Therefore,this shows that the equilibrium of K-Mg-SiO₂ system has been established in the reservoirs.However,the ion-exchanging equilibrium between Na-K,Na-Li failed to achieve,and the ratio of Na-K to Na-Li in geothermal fluids remains basically unchanged during the ascending processes.However,the relationship between Li-Mg ratio and reservoir temperatures still can not be established.In the geothermal fields without the direct influence of magmatic degassing,the circulating depth of geothermal fluids is relatively shallow and the reservoir temperature is relatively low（below 200℃）,so the conductive cooling process may not be enough to influence the accuracy of the K-Mg geothermometer.Therefore,K-Mg geothermometer and chalcedony geothermometer are more suitable for the estimates of the reservoir temperatures in this section.Nevertheless,the Na-K geothermometer and Na-Li geothermometer have a high estimating values,while the Li-Mg geothermometer are lower.Although the bicarbonate geothermal fluid may have sufficient CO₂ recharge in the reservoirs so that K-Ca ratio can reach the equilibrium with relatively low reservoir temperature,the Na-K equilibrium is not easy to achieve.Therefore,the Na-K-Ca geothermometer is also not suitable for the estimation of reservoir temperatures in this kind of geothermal system.3.In the case of the Xinyi and Fengshun geothermal fields located in the coastal area of Guangdong Province,how to establish hydrochemical equilibrium of different geothermometers via dissolving granite in low-temperature reservoirs is studied.In the low temperature reservoirs formed by felsic rocks,the interacting extents of water-rock is relatively low,and the main hydrochemical type of geothermal fluid is“soda water”.Although this kind of geothermal fluid is far from the water-rock equilibrium,some minerals in the reservoirs may still reach the dissolving equilibrium.Via dissolving primary minerals,the geothermal fluid are more likely to reach equilibrium with chlorite and chalcedony,and the ion-exchanging equilibrium between K-feldspar and muscovite or kaolinite which controls the concentration of K and Al in geothermal fluid.However,geothermal fluids are difficult to reach equilibrium with Na and Ca bearing minerals（e.g.plagioclase）,so different water samples may have various concentration of Na and Ca due to the different degrees of dissolving granite.In addition,the variation of Li concentration in geothermal fluids of low temperature reservoirs is relatively large,and with the poor correlation with reservoir temperatures.Therefore,the geothermometers related to Li is not suitable for the temperature estimating of low-temperature reservoirs.When the FixAl method is utilized to estimate the temperature of low-temperature reservoirs,it will be better to choose the forced equilibrium mineral as K-feldspar rather than albite.4.Taking the Rehai geothermal field in Yunnan and Kangding geothermal field in Sichuan province as the cases,the main changes of chemical composition of geothermal fluid after undergoing the different cooling processes were studied,including their influence for the accuracy of geothermometers.In the natural geothermal fields,geothermal fluids can always retain the hydrochemical informations in the reservoirs if they only undergo adiabatic cooling process to the surface,even some deposition will appear in the outlets of thermal springs.The extents of water-rock interaction will in the different reservoirs will be the key factors that influences the accuracy of classical geothermometers.If the geothermal fluids ascend to the surface slowly,the Mg concentrations will increase easily due to reacting with the rocks so that the estimates of K-Mg geothermometer will reduce.Based on the different hydrochemical types of groundwaters,the mixing process may generate a greater impact on the hydrochemical composition of the original geothermal fluid.Therefore,when using a hydrochemical thermometer to estimate the temperature in reservoirs,we should not neglect the hydrochemical composition of groundwaters which involved in the mixing process iin that they may have a significant influence on the hydrochemical composition of the original geothermal fluid.For the Li-related geothermometers,the Na-Li geothermometer seems to be suitable for the high-temperature reservoirs,in this section,Li-Mg geothermometer is invalid for the geothermal fluids in Rehai geothermal field and Kangding geothermal field.