Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 Geochemistry of Potential High Temperature Geothermal Resources in Kangding, Sichuan, China Zihui CHEN 1, Yi XU 2 and Keyan ZHENG 3 1 China Institute of Geo-Environmental Monitoring, Beijing 100081, China 2 Ganzizhou Kangsun Geothermal Co. Ltd., Kangding 626000, China 3 Geothermal Council of China Energy Society, Beijing 100081, China 1 13810757913@163.com Keywords: geochemistry, geothermal, Kangding ABSTRACT High temperature geothermal fluid with wellhead temperature of 174 C was found in recent year in Kangding County, Sichuan Province, China. The high temperature geothermal resources in China concentrate upon Himalayan Geothermal Zone in Tibet. Kangding is located close to the northeastern margin of Himalayan Geothermal Zone. Kangding geothermal field is fractured reservoir within granite and a few sandstone of Permian System. Geothermal water in the reservoir is Cl.HCO 3 -Na type with total dissolved solids of 2,457-3,114 mg/l. The geothermal water is mineral water with higher concentrations of silica 186.2mg/L, fluorine 12.71mg/L and lithium 7.965mg/L. The lower molecule ratio of Na/K as 6.07-19.66 means a higher reservoir temperature. Geothermometry also shows higher K-Mg temperature and silica temperature as 179.8 C and 175.1 C respectively. These geochemical features are similar with the characteristics of high temperature geothermal resources in Yangbajain and Yangyi geothermal fields in Tibet. This discovery enlarges the distribution range of high temperature geothermal resources in China. It is hopeful to use the resources for geothermal power generation and integrated utilization in Kangding region. 1. INTRODUCTION Kangding County is the capital of Ganzi Tibetan Autonomous Prefecture, which is located in western Sichuan province, latitude N29 55-30 10, longitude E101 42-102 00. It s about 300km highway mileage that southwesterly direction from Chengdu to Ya'an, and then turn to the west to Kangding. The Mediterranean Himalayan Geothermal Zone is one of the global high temperature geothermal zones. Its eastern section traverses Tibet which is a concentrated zone of high temperature geothermal resources in China. Himalayan Geothermal Zone in Tibet is generally from west to east along the river Yarlung Zanbo. There are many geysers, boiling springs, boiling fountains, as well as hydrothermal explosions and hydrothermal alteration and other high-temperature geothermal manifestations. Its east arrived in Ganzi Tibetan Autonomous Prefecture, through Batang, Litang with boiling springs occurred, then arrived in Kangding County with Guanding hot springs of 94 C there (Figure 1). Figure 1: Kangding located in Himalayan Geothermal Zone 1
Himalayan Geothermal Zone turns to south from Kangding, extends towards the western Yunnan province, to the south to Tengchong and Longling. There are 10 boiling springs in Tengchong, which has the largest number of hot springs in China. Kangding Guanding hot springs belong to Xiaoreshui geothermal field. Geothermal well has been drilled with wellhead blowout temperature 174 C in recent years (XU and LIANG, 2013). Here is the northeastern end of the Himalayas Geothermal Zone, close to the outer edge of the zone. By this study using geochemical method, Kangding hot spring area has been confirmed belonging to high temperature geothermal field just similar as Tibetan Yangbajain and Yangyi geothermal fields. 2. KANGDING GEOTHERMAL GEOCHEMISTRY Various water samples were collected in Kangding Xiaoreshui geothermal field and surrounding area to conduct chemical analysis detecting of full components (ZHENG and CHEN, 2013). These are hot springs with temperature higher than 60 C, warm springs with temperature between 33 C to 60 C, geothermal wells with depth at 109-267 m, shallow groundwater with depth less than 20 m, cold springs with temperature lower than 15 C, river and local rain water. The purpose is to find out regularity that different sources of water showed different water types as listed in Table 1. The various water types show much clear regularity usually than a table of concentration of cation and anion. Table 1: Water chemistry and TDS of different waters in Kangding Xiaoreshui geothermal area Water Type Chemical type Total dissolved solids(mg/l) Geothermal well Cl.HCO 3 -Na,HCO 3.Cl-Na,HCO 3 -Na 2,457-3,114 Hot spring HCO 3.Cl-Na,HCO 3 -Na, 1,024-1,372 Warm spring HCO 3 -Na.Ca 446-1,223 Shallow groundwater HCO 3 -Na,HCO 3 -Na.(Ca) 284-388 Cold spring HCO 3 -Na.Ca 1,188-401 River HCO 3 -Ca,HCO 3.SO 4 -Ca.Mg*,HCO 3 -Ca.Na 38-176 Rain HCO 3 -Ca 135 *one of samples has a high percentage of SO 4, but the concentration is not high, it was effected by local factors The above regularity shows more clearly in the Langelier-Ludwig diagram (Figure1). Geothermal well spots are among the top of the diagram; Then the hot spring spots located below the geothermal wells; Position of warm spring slightly lower than hot (a warm spring spot located in cold water zone of the bottom right of the diagram); Some shallow groundwater spots crossed with warm spring, but overall they slightly lower than the warm spring spots. These situations mentioned above hold on the upper part of the diagram, that sodium is main cation. The lower half of the diagram is cold water type, calcium is main cation. The rain spot located at the bottom right corner, which is the most typical cold water; Stream spots position slightly above the rain and have a larger distribution; Distribution range of cold spring water spots are also large, but overall they are higher than the surface water. The horizontal axis is anionic character, the right end is bicarbonate advantage, while the left end is chloride advantage. Geothermal water is containing more chloride anion than cold water (shallow groundwater and surface water). Especially in high temperature geothermal water, chloride anion has absolute advantage. Another anion in water is sulfate, but only appear a higher percentage in one stream sample, while its content is not high. The total dissolved solids in the sample was only 118mg/L (lower than the rainwater collected), showing a partial effect. Figure 2: Langelier-Ludwig diagram of different waters in Kangding Xiaoreshui geothermal field (Zheng & Chen, 2013) 2
The purple line in Langelier-Ludwig diagram (Figure 2) linked rainwater and the highest temperature geothermal well ZK201 water, which is called the mixing line of cold and hot water (Langelier & Ludwig, 1942). To confirm the mixing relationship between cold and hot water, a Langelier - Ludwig section diagram (figure 2) was cut out along the purple line vertically from figure 1. Here labeled each chemical type of water samples and the relationship between the total dissolved solids and them, which constitute two crossed lines in figure 2. In the middle of Figure 2 a warm spring point and the cold water point at the bottom constitute a true "mixing line", the water types and total dissolved solids are consistent with the mixing rules. Guanding hot springs are the source of hot water and the source of cold water is local rain water and surface waters. All kind of natural water body found in Xiaoreshui geothermal field is made up by a mix process of different proportion of hot and cold waters, from the hot and cold sources mentioned above. In addition, in upper part of figure 3, there seems to be another kind of mixed relationship between the highest temperature well ZK201 and Guanding hot springs. And there should be the highest total dissolved solids of hot water to form the most reasonable heat source. Perhaps this explains why ZK201 well could not serve as the heat source representative in Xiaoreshui geothermal field. That is to say, Xiaoreshui geothermal field should have hot water of highest concentration in deep reservoir. Figure 3: Langelier-Ludwig section diagram of Kangding Xiaoreshui geothermal field (Zheng & Chen, 2013) 3. COMPARISON WITH THE HIGH-TEMPERATURE GEOTHERMAL FLUIDS OF TIBET Hot water yielded in geothermal wells shows the highest temperature of Kangding geothermal field. The natural hot springs and fountains in this area have been mixed with local cold waters (rain, surface water and shallow groundwater). Therefore we use data of geothermal wells to make comparison among the Kangding Xiaoreshui geothermal field and Tibetan Yangbajain, Yangyi and Nagqu geothermal fields. 3.1 Water Type and Total Dissolved Solids We listed the comparison of water type and total dissolved solids among Kangding Xiaoreshui geothermal field and three geothermal fields in Tibet (Table 2). Tibet Yangbajain (Geothermal Geological Team of Tibetan Geological Bureau, 1983) and Yangyi (Geothermal Geological Team of Tibetan Geological Bureau, 1990) are high-temperature geothermal fields. Here also lists the data of Nagqu medium-temperature geothermal field (Geothermal Geological Team of Tibetan Geological Bureau, 1989) (maximum temperature of drillhole is 115 C) for expanding the scope of contrast. Table 2: Comparison of water type and TDS for Kangding and Tibetan high-temperature geothermal fields Kangding Xiaoreshui Yangbajain Yangyi Nagqu Water chemical type Cl.HCO 3 -Na Cl-Na HCO 3.(Cl)-Na HCO 3 -Na TDS (mg/l) 2,457-3,114 1,110-2,827 1,064-1,303 1,591-1,722 We made contrast of Langelier-Ludwig diagram for four geothermal fields (Figure 4). Each field has their own mixing line, reflecting the mixed relationship of local hot springs, cold springs and surface water. From the view of water type, sodium is dominant cation for hot water from all four geothermal fields, located at the top position of Langelier-Ludwig diagram. However, the anion characteristics for the four fields are different. The content of chloride has absolute advantage in the anion of Yangbajain geothermal hot water, which is located in the top-left side of the diagram; the situation in Kangding geothermal field is close to Yangyi geothermal field, located at the middle-top of the diagram. The percentage of chloride content is slightly higher than bicarbonate in Kangding, while the percentage of bicarbonate content is slightly higher than chloride in Yangyi. Nagqu geothermal hot water located at the top-right side of the diagram, bicarbonate has an absolute advantage for its anion. 3
Figure 4: The comparison of water chemical type of high temperature geothermal resources (Zheng & Chen, 2013) Kangding (Upper left), Yangbajain (upper right), Yangyi (lower left), Nagqu (lower right) Comparing the total dissolved solids (TDS) of geothermal water for four fields, Kangding geothermal water has the highest value in 2,457-3,114 mg/l; Secondly Yangbajain geothermal water is in 1,110-2,827mg/L with the average of 1,509mg/L; Nagqu geothermal water is similar with Yangbajain, in 1,591-1,722mg/L; Yangyi has the lowest value in 1,064-1,303mg/L. For the water chemistry type and TDS characteristics of geothermal water discussed above, we can draw the following conclusions: (1) The water chemical type of high temperature geothermal resources in the world is sodium chloride type, its origin relevant to modern volcano or shallow magma chamber. So the sodium chloride type of Yangbajain geothermal water, basically belongs to the deep geothermal reservoir characteristics. Yangyi and Kangding geothermal waters are mixed with bicarbonate in sodium chloride water, it means relatively shallow water recharge. This kind of geothermal field maybe can meet sodium chloride type water in deep section. Nagqu geothermal field does not have the characteristics of deep sodium chloride water. (2) From the view of TDS, Kangding geothermal water has the highest content, which seems to indicate the meteoric water recharge is small; Yangyi geothermal water has the lowest value, which indicates the meteoric water cycle recharge is relatively rich. Yangbajain and Nagqu geothermal fields are in the middle case of cold water recharge. 3.2 Standard Components of Geothermal Water Geothermal water generally contains some special ingredients, such as meta- silicate, fluorine and lithium, which are much higher than in various cold waters. It is called standard components for geothermal water. The standard components of Kangding geothermal field and 3 Tibetan geothermal fields are listed to compare in Table 3. 4
Table 3: Comparison of standard components for Kangding and Tibetan geothermal fields Kangding Yangbajain Yangyi Nagqu Meta-silicate (mg/l) 102.4-242.1 150.8-441.2 200.2-393.3 117.3-130.7 Fluorine (mg/l) 4.20-12.71 4.46-34.22 9.50-22.04 8.17-9.12 Lithium (mg/l) 6.715-7.965 7.64-29.35 4.80-10.00 3.06-3.28 Nagqu geothermal field is middle-temperature geothermal resources. Its contents of meta-silicate, fluorine and lithium are also obviously at a low level. The content of meta-silicate, fluorine and lithium for 3 high temperature geothermal fields has reached a mineral water standard that can be named as silicon water, fluorine water and lithium water. So either for meta-silicate or fluorine or lithium we can see the same regularity. Yangbajain has the highest content (Piovesana et al, 1987). Then Secondly for Yangyi geothermal field. While Kangding geothermal field is relatively slightly lower. From the consistent rules above, the geothermal potential of Kangding Xiaoreshui geothermal field is just little below Yangbajain and Yangyi. 3.3 The Geochemical Temperature Scale Water sample collected from wellhead retains information of deep geochemical environment. Geothermometry is a method to recover the temperature "memory" for deep circulation. So it can be used to infer and calculate underground temperature. For the ratio of potassium and magnesium when at water/rock equilibrium, using K/Mg geothermometer which deduced by thermodynamic equilibrium equation, can calculate the temperature potential (Giggenbach, 1986). It is also called "drilled temperature", on behalf of the temperature at relatively not too deep depth. In addition, using the silica content in the water can find out the temperature of deep reservoir, according to the silica solubility curve under different temperature conditions actually. When the geothermal water flows out from the geothermal reservoir with temperature decline, the silica will not precipitate out immediately. So it can "memorize" the temperature of deep circulation. The calculation of geothermometry for Kangding Xiaoreshui geothermal field and high temperature geothermal fields in Tibet are listed in table 4. It is obvious that all temperature data of Nagqu mid-temperature geothermal resources are relative to the lowest, so we only choose 3 high temperature geothermal fields to make comparison below. Table 4: Geothermometry for Kangding and Tibet high temperature geothermal fields Kangding Yangbajain Yangyi Nagqu Maximum temperature at wellhead 174 C 200 C 190 C 110 C Maximum temperature at well bottom 208 C 329.8 C 207.16 C 115 C K/Mg temperature 133.5-179.8 C 143.2-202.6 C 125.4-168.4 C 121.1-126.6 C Silica temperature 124.3-175.1 C 145.7-219.1 C 170.7-210.0 C 146.3-152.7 C (1) For measured temperature at wellhead, Yangbajain geothermal field is the highest, secondly Yangyi geothermal field, while Kangding geothermal field shows the minimum. (2) For measured temperature of downhole, Yangbajain geothermal field is the highest, while Yangyi and Kangding geothermal fields are quite similar. However, the highest temperature in Kangding is measured at 267.16 m depth of a shallow well, but Yangyi s temperature is measured at a deeper well. (3) For K/Mg temperature, Yangbajain geothermal field is the highest, secondly Kangding geothermal field, while Yangyi geothermal field is in the third. (4) For silica temperature, Yangbajain geothermal field is the highest, secondly Yangyi geothermal field, while Kangding geothermal field is in the third. From the comparison above, Yangbajain geothermal field has the highest potential currently, Yangyi and Kangding geothermal fields have the similar potential, the Yangyi geothermal field may be slightly better. 3.4 Chemical Ratio In the upflow process from deep reservoir, geothermal water was diluted by infiltration of meteoric precipitation. Its chemical composition content is reduced, while the molar ratio between various components is not influenced. Chemical ratio for Kangding geothermal field and Tibetan high temperature geothermal field show in table 5. 5
Table 5: Chemical ratio for Kangding and Tibetan high temperature geothermal fields Kangding Yangbajain Yangyi Nagqu Na/K 6.07-19.66 6.54-14.23 17.01-32.03 36.02-37.07 Cl/B undetected 2.72-3.00 2.10-3.75 5.81-6.71 Cl/F 28.1-66.3 7.05-19.63 1.07-8.16 12.17-14.46 Cl/SiO 2 6.17-14.10 3.30-7.89 0.10-1.57 3.12-3.34 (1) Na/K ratio is related to the temperature level that the lower Na/K represents the higher temperature of geothermal reservoir. Yangbajain geothermal field has the minimum of Na/K, secondly Yangyi geothermal field, and Nagqu geothermal field has the highest Na/K, which totally correspond with the temperature characteristic of these geothermal reservoir. In generally, Na/K ratio in Kangding geothermal field is between Yangbajain and Yangyi geothermal fields, which illustrate abundant potential of high temperature geothermal resources. (2) Cl/B ratio is related to the cause of formation of geothermal field that each of fields has a basically consistent Cl/B. The range of Cl/B of each geothermal field in Tibet is not big. Unfortunately that Kangding geothermal field did not detect boron content. (3) Cl/F ratio is related to the solubility of CaF 2. While cold water invaded the system, the solubility of CaF 2 will be increased, so Cl/F will be decreased. It is hard to compare for the long distance among four geothermal fields in Table 5. We took Cl/F detected result of ZK325, ZK354 and ZK356 wells of Yangbajain geothermal field in 1991 and 1994, found out they had a similar regular change. The Cl/F has decreased a lot than 3 years before (Table 6), which illustrated the intensive exploitation lead to the cold water recharge increased. The value of Cl/F of ZK201 well in Kangding geothermal field in 28 April 2012 and 8 June 2013 are 50.54 and 44.47 respectively, the decrease proved an increased recharge of cold water. We should pay more attention to such trend. (4) Cl/SiO 2 ratio is mainly subjected to chloride concentration and it will be decreased when cold water invasion to geothermal system. Four geothermal fields separated far and wide so it is difficult to compare them. We took Cl/SiO 2 detected result of ZK325, ZK354 and ZK356 wells of Yangbajain geothermal field in 1991 and 1994, found out they have a similar regular change. The Cl/SiO 2 has decreased than 3 years before (Table 6), which declared the intensive exploitation lead to the cold water recharge invaded to the shallow geothermal reservoir. The value of Cl/SiO 2 of ZK201 well in Kangding geothermal field in 28 April 2012 and 8 June 2013 are 9.78 and 7.74 respectively, the decrease proved increased recharge of cold water. We should pay more attention to such trends. Table 6: Cl/F and Cl/SiO 2 ratio change in Yangbajain geothermal field in 1991 and 1994 Cl/F Cl/SiO 2 1991 1994 1991 1994 ZK325 well 17.03 7.05 7.89 5.81 ZK354 well 18.03 7.83 7.33 6.85 ZK356 well 19.63 7.57 7.76 7.06 Average of 3 wells 18.23 7.48 7.66 6.57 The change after 3 years Sharply decreased decreased The meaning of change Cold water invasion to shallow geothermal reservoir 4. CONCLUSIONS Kangding Xiaoreshui geothermal field in addition to measured temperature is already high temperature geothermal resources, the geochemistry demonstrates the same features. In comparison, geochemical comparison of Kangding Xiaoreshui geothermal fields with known geothermal fields in Tibet showed Yangbajain has the strongest high-temperature geothermal resource characteristics, then Yangyi geothermal field, and then Kangding Xiaoreshui geothermal field (also occasional exceptions), but they all certainly much higher than Nagqu medium-temperature geothermal fields. (1) Water chemical type: Typical water chemistry of high temperature geothermal fields in the world is Cl-Na type. Yangbajain is of this type. The second is Cl.HCO 3 -Na type, like Kangding Xiaoreshui. Then followed by HCO 3.(Cl)-Na type of Yangyi geothermal field. (2) Standard components: The geothermal water of Yangbajain geothermal field has the highest content of silica, fluorine and lithium. It is better than Yangyi geothermal field, followed by Kangding Xiaoreshui geothermal field. 6
(3) Geothermometry: Either measured temperature at wellhead and downhole or K/Mg temperature or silica temperature, basically consistent with the highest Yangbajain, then Yangyi followed (only K/Mg temperatures below Kangding), while Kangding Xiaoreshui in third. (4) Na/K ratio: The low value reflects a higher temperature level in geothermal reservoir. Yangbajain has the lowest ratio, followed by Yangyi, while Kangding Xiaoreshui in third. Overall, the geochemical characteristics of Kangding Xiaoreshui geothermal field are similar to the high-temperature geothermal resources of Yangbajain and Yangyi. Although it is not as strength of Yangbajain, however, at least it has the potential to slightly inferior to the Yangyi geothermal field. Geochemical characteristics of Kangding geothermal fields fully demonstrated the features of high temperature geothermal resources. Exploitation of Xiaoreshui high-temperature geothermal fields is expected to achieve geothermal power generation and tail water integrated utilization. It will enhance local economic development and benefit the majority of people in Kangding. REFERENCES Geothermal and Geological Team of Tibetan Geological Bureau: Geothermal geological survey report of Yangbajain, Tibet, (1983), pp.38. (in Chinese) Geothermal and Geological Team of Tibetan Geological Bureau: Exploration Report of Yangyi Geothermal Field of Damxung County, Tibet, (1990), pp.120. (in Chinese) Geothermal and Geological Team of Tibetan Geological Bureau: Detailed Geological Survey Report of Nagqu Geothermal Field of Nagqu County, Tibet, (1989), pp.86. (in Chinese) Giggenbach, W.F.: Graphical techniques for the evaluation of water/rock equilibration conditions by use of Na, K, Mg and Ca contents of discharge waters, Proceedings, the 8 th New Zealand Geothermal Workshop. (1986) 45-47. Langelier, W.F., Ludwig, H.F.: Graphical methods for indicating the mineral character of natural waters. J. Am. Water Works Assoc. 34 (1942) 335. Piovesana, F., Scandiffio, G., Zheng, K., Zuppi, G.M.: Geochemistry of thermal fluids in the Yangbajain area (Tibet/China). Proceedings, International Symposium on the Use of Isotope Techniques in Water Resources Development, IAEA-SM- 299/136, (1987) 47-70, International Atomic Energy Agency. Xu, Y., Liang, T.L.: Exploration Discovery of High Temperature Geothermal Resources in Kangding County, Sichuan Province, Exploration and Development of High Temperature Geothermal Resources in China, (2013), 55-61. (in Chinese with English abstract) Zheng, K.Y., Chen, Z.H.: A Comparison of Fluid Geochemistry Between Kangding and Tibet High Temperature Geothermal Resources. Exploration and Development of High Temperature Geothermal Resources in China, (2013), 82-89. (in Chinese with English abstract) 7