PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 11-13, 2013 SGP-TR-198

THE CHEMICAL PROPERTIES OF YANGYI HIGH TEMPERATURE GEOTHERMAL FIELD IN TIBET ,CHINA

Zheng Xiuhua, Duan Chenyang, Liu Haiyang

China University of Geosciences (Beijing)

29 Xueyuan Road, Haidian District, Beijing, 100083, P.R. China e-mail: xiuhuazh@cugb.edu.cn

ABSTRACT

Yangyi high temperature geothermal field was explored in the 1990’s and now is being developed for power generation with 40 MWe of capacity in Tibet. Chemical Properties from 8 boreholes were analyzed to evaluate the resources and predict the scaling potential of this geothermal field. The results show that the geothermal water presents alkalescence with 7.5-9.5 of pH and the total dissolved solids (TDS) are low, and vary little ranging from 1.4 to 1.8g/l. The geothermal water is characterized by a dominant HCO3–Na composition according to the Piper diagram Compared the concentrations of some compositions in the thermal water with that in the cold spring around the field, it can be found that the concentrations of Li

+, F

-, Cl

- (especially Li

+) in the

thermal water are higher, which indicates the thermal water supply is from deep water with the replenishment from atmospheric precipitation. Taking into account the cation and SiO2-temperature geothermometers, the temperature of the reservoir

may be estimated to be about 180~210℃.

INTRODUCTION

With increasing energy demand and the decreasing traditional energy supplement, renewable energies play an important role. As a renewable energy, more attention is focused on the geothermal energy. In 1970s, the first geothermal power plant was constructed Yangbajing, Tibet Province, China, which was great contribution to overcome Winter Electrical Shortage in Lasha District, and get huge social and economic benefits. Nevertheless, as the rapid economic development and more demand for electricity, the Yangbajing generating capacity is no longer adequate for the need. Yangyi Geothermal field is another high temperature geothermal field after Yangbajing Geothermal field. Though the physical exploration, chemistry exploration, drilling exploration and tests, it has been proved that Yangyi

Geothermal field has the recourse condition to be exploited. This article is mainly interpretation the hydrochemical characteristics during exploration on the basis of the hydrochemical analyze sampled from the 8 drillholes in Yangyi Geothermal field.

SITE DESCRIPTION

Yangyi Geothermal field, located at west of Yangyi village, 72Km to Lasha City and 55Km north-east to Yangbajing, is situated in the south of Yangbajing Basin. The faults in the field can be divided into three groups, faults having S-N trend predominate in the Yangyi area; faults having S-N trend manifested as three parallel aligned valleys, namely Qialagai River, Nangzeng River and Bojiemu River, stagger the N-E trend faults and control the generation of hydrothermal activity; the younger faults having N-W trend stagger the others’ faults (Liao, 1989). The three fracture structures mentioned above control the landscape and distribution of hydrothermal activities of Yangyi Geothermal field. Since the Miocene, contiouous fault structure activity dominate multiple hydrothermal activities. Thermal deposit base is mainly dominated by granite porphyry, covered by Miocene volcanic rocks, which thickness is 40-50m. The volcanic rocks is made up of trachyandesite, pyroxene trachyandesite and biotite, outcropped from south to north in the field and interbed with pyroclastic rock, inclined to northwest 30-40°. In the east and west, the volcanic rocks are covered by Pleistocene clay (Q1) and clastic sediments (Q2 and Q8) . On the basis of temperature, pressure and outflow tests, meanwhile, referring to ground noice velocity power spectrum main frequency and amplitude contour map and minimal apparent resistivity contour map, integrating with geothermal features, Yangyi Geothermal field can be divided into three resource areas, high temperature area, intermediate temperature area and low temperature area. High temperature area locates between F5 and Wurenzhema, from Qialagai River in the north to Bojiemu River in the south. The three areas distribute

just like concentric elliptical on the plane, semimajor axis nearly south-north and the high temperature area lies the centre of the plane(Zhu 1992).

Figure 1: Map of geological and alteration zonation

in Yangyi geothermal field. 1: montmorillonite ; 2: kaolinite ; 3:

montmorillonite &α - christobalite; 4:

alunite;5: fiorite ; 6: sinter; 7: spring; 8: drillhole; 9: fault.

SAMPLES AND METHODS

Samples used in the present study were those collected from 8 drill holes (ZK200, ZK203, ZK204, ZK206, ZK301, ZK302, ZK401 and ZK501) and surface water sampled from the Qialagai River and Bojiemu River.

Temperature, pH and electrical conductivity (EC) of the water samples were measured on-site. The groundwater samples were filtered with a cellulose nitrate membrane with a pore size of 0.22μm. The samples for cation analysis were later acidified to pH< 2.0 by adding ultrapure HNO3. The samples were collected in 110-ml high-density polypropylene bottles, sealed with a double cap and stored in a refrigerator until analysis(Zhang el at., 2008).

Major cation concentrations were measured by inductively coupled plasma-optical emission spectrometry, and anion concentrations by ion chromatography.(Andrés el at., 2011) Silicon and minor element concentrations were measured by inductively coupled plasma mass spectrometry.

RESULTS AND DISCUSSION

Major chemical characteristics of geothermal water

The main physical and chemical characteristics of the geothermal waters are shown in Table 1. It shows that most of the waters are neutral with an alkaline tendency (pH 7.5-8.5). The change in salinity isn’t significant, rang from 1.4 g/l to 1.8g/l. It exists high

concentrations of Na+, Li

+, Cl

-、SO42-

, HCO3-, CO3

2-,

F- and SiO2, trace elements in the water include Fe

2+,

Fe3+

, NH4+, Cu

2+, Pb

2+, Zn

2+, CO2 is the main gas

component in these samples(Liang el at. 1990). The geothermal water is characterized by a dominant HCO3–Na composition according to the Piper diagram(Fig.2). The dominant cation is sodium, which accounts for 85% of total cation content. Bicarbonate accounts for 60% of total cation content is the mainly anion.

Figure 2: Piper diagram of geothermal water

samples in Yangyi. According to the formula by Giggenbach(1991), Cl-SO4-HCO3 triangular diagram can explain properties of major anions in current geothermal water and express different parts of the geothermal system or links among the different types of geothermal systems. The Cl-SO4-HCO3 triangular diagram of Yangyi Geothermal field is shown in Fig. 2. Compared with Cl

- and SO4

2+,the content of HCO3

- is

more plentiful. The water types plot in the middle of

external water area along the Cl-HCO3 axis. which is the sign of external water properties(Opondo, 2008). This could suggest that the waters are formed by deep steam ascend to near surface condensate though dilution of meteoric water with the absorption of carbon dioxide gas. According to the formula by Giggenbach(1991), Cl-SO4-HCO3 triangular diagram can explain properties of major anions in current geothermal water and express different parts of the geothermal system or links among the different types of geothermal systems. The Cl-SO4-HCO3 triangular diagram of Yangyi Geothermal field is shown in Fig. 2. Compared with Cl

- and SO4

2+,the content of HCO3

- is

more plentiful. The water types plot in the middle of external water area along the Cl-HCO3 axis. which is the sign of external water properties (Opondo, 2008). This could suggest that the waters are formed by deep steam ascend to near surface condensate though dilution of meteoric water with the absorption of carbon dioxide gas.

Sources of geothermal water

According to the comparison of various components in geothermal water and surface water (Table 1 and 2), Cl

-, F

-, SO4

2-, K

+, SiO2, Li

+, Li

+/Na

+ in geothermal

water samples differ widely compared with the surface water samples, especially the content of Li

+,

which can reach 7.01 mg/l.

Figure 3: Relative Cl, SO4 and HCO3 conents of thermal fluid from wells in Yangyi.

Geothermal water characteristics components are determined relative to surface water, according to Table 2, the average lining values of Na

+, Li

+, F

-, Cl

-

are greater than 34.12, which can be the special chemistry components of this geothermal field(Qian el at., 2007). The content of Cl

-, F

- and Li

+ in the

geothermal water is abnormally high relative to the surface water. There is a strong link among Cl

-, F

-

and Li+

on geochemical behavior, these primarily enrich in late magmatic differentiation or residue in hydrothermal. Granite reservoir leached by reservoir water is unlikely to cause particularly high content of Li+, the average value of Li

+/Na

+ in igneous is just 0.00081,

however, the value in this area can reach 0.0157, even despite the effect of CO2, high temperature and pressure condition. This situation is not only explained by leaching on surrounding rock, it is more likely to be influenced by direct effect of magma composition (Cidu & Bahaj, 2000). So it is proved that reservoir water supply is part from the deep water (primitive water or magmatic water). However, only deep water can’t meet the need of the reservoir water supply, and the samples present the external water properties, so meteoric water is also an important part to replenishing the reservoir water.

Geothermometer

Numerous geothermometer have proposed by various workers. Geothermometer can be divided into two groups. One is silicon geothermometers, such as quartz, chalcedony(Pang & Wang, 1989). The other is cation geothermometer. Cation geothermometers establish based on the relationship between the exchange between hot water and cation in the solid material and temperature, like Na-K, K-Mg, Na-K-Ca (Huang el at., 2010).

Figure 4: Na-K-Mg diagram of thermal water samples in Yangyi Geothermal Field.

As the calculated temperatures are shown in Table 4, the temperatures estimated by Na-K-Ca geothermometer are far higher than other temperature calculated by other geothermometers. It can be

attributed to the concentration variety which the reservoir water mixes with cold water, so that Table 4: Reservoir temperature estimated with different geothermeters(Pirlo 2004).

Number Sample

temperature(℃) quartz (℃) chalcedony

(℃) Na-K (℃) Na-K-Ca (℃)

ZK200 70 208.5882 192.2928 192.8981 338.3925

ZK203 61 205.4868 188.6259 191.2449 326.7541

ZK301 79 210.0052 193.9716 208.2006 364.6736

ZK302 77 170.6614 148.1689 168.1234 308.768

ZK401 79 177.3641 155.8545 184.1939 327.361

ZK204 72 183.116 162.4878 173.2854 321.6481

ZK206 21 183.3005 162.7012 182.8881 327.5803

ZK501 77 208.5882 192.2928 189.0577 344.9514

influencing the Na-K-Ca geothermometer (Wu &Sun, 2001). Boling emissions CO2, HCO3

- is

converted to CO32-

, thereby leads to CaCO3

precipitation, and the cost of soluble Ca2+

will bring to the exorbitant temperature calculated temperature. Before using cation geothermometer, mineral-fluid judgment is required, Na-K-Mg triangular diagram is a common method. Na-K-Mg triangular diagram divids water into three styles: complete equilibrium water, partial equilibrium water and immature water (Powell & Cumming,. 2010). It can be used to judging the fluid with material attaining equilibrium or not. The reservoir temperatures calculated by quartz, chalcedony and Na-K are close, it illustrates that underground hot water after equilibrium impacts little by the shallow water during ascending. The hot water which has flown to the surface can represent the reservoir characteristics basically. Whether the samples appropriate to using Na-K equilibrium temperature usually corresponds to the water in the area is immature or not. The samples in Yangyi are all located at partial equilibrium water area, so Na-K equilibrium temperature can evaluate the temperature in Yangyi. (Pirlo 2004)So, SiO2 and Na-K geothermometers are better methods to estimating temperature in Yangyi Geothermal field. For different SiO2 geothermometers, the temperature calculated by quartz geothermometer is highest, which reflect the highest temperature of the reservoir. Reservoir temperature estimated by chalcedony geothermometer is low, so these two geothermometers give the possible reservoir temperature: 180-210℃.

CONCLUSION

Yangyi high temperature geothermal field is another high temperature geothermal field found in Tibet after Yangbajing Geothermal field, which possesses abundant resources. It can ease greatly power

problems in Tibet after utilizing Yangyi geothermal field. Reservoir water of Yangyi geothermal field contains high concentrations of Na

+, Li

+, Cl

-, SO4

2-, HCO3

-,

CO32, F

- and SiO2, CO2 is the major gas composition.

The reservoir water is mainly meteoric water heated after entering the reservoir though fault and fissure, addition some ascended deep circulating water. The reservoir temperature is 180-210℃ calculated by the geothermometer. However, the temperature estimated by Na-K-Mg geothermometer is excessive temperature. this is due to the water is in partial equilibrium, some ions loss owing to cold water interfusion, thus the accuracy of Na-K-Mg geothermometer declines.

REFERENCES

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