Tracer Response of Injection Test in Hot Spring Fluid Layer at Minami-Izu Geothermal Field, Shizuoka, Japan

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1 GRC Transactions, Vol. 37, 2013 Tracer Response of Injection Test in Hot Spring Fluid Layer at Minami-Izu Geothermal Field, Shizuoka, Japan Norio Yanagisawa 1, Kazuo Matsuyama 2, Yasuto Takeda 2, Kazuo Tomita 2, Kasumi Yasukawa 1, and Keiichi Sakaguchi 1 1 Institute for Geo-Resources and Environment, AIST, Ibaraki, Japan 2 Koyo Electric Corporation, Tokyo, Japan n-yanagisawa@aist.go.jp Keywords Hot spring, tracer, injection, fault, return ratio, optical fiber, Minami-Izu ABSTRACT Tracer test is carried out to survey flow properties in high temperature hot spring fluid layer about 150 meter depth at Minami-Izu geothermal field, Shizuoka, Japan. About 500g of uranine was used for tracer and injected on 16 September, We monitored tracer appearance at 5 wells using optical fiber system and lab spectrometer. After 9 hours from tracer injection, first tracer appeared at Daigaku-yu (K-13) well about 70 meter distance SSE from the injection well along large fault. The tracer concentration rapidly increased and peaked at three days after injection. And the return ratio is estimated about 30%, similar to several EGS test fields. After 5 days, tracer appeared at a monitoring well about 600 meter depth and this suggests flow path to deeper reservoir. In Tamagawa-yu (K-11) about 150 meter from injection well, tracer appeared in early October. This tracer response shows the natural flow of hot spring fluid direction to east. Introduction Izu peninsula exists south of Mt. Fuji and about 100 km SW from Tokyo and belongs to the Philippine Sea Plate. On Hachijo island, one of the volcanic islands, a small flash geothermal power plant (about 3,300kW) started power generation in On the east side and south edge of Izu peninsula, there are many high temperature (about 100 degree C) hot springs from about 150 meters depth and many resort hotels. In these areas, there is not geothermal power plant in spite of the high geothermal potential due to resistance of hot spring owners. But after the nuclear power plant accidents that followed the earthquake on 11 March, 2011, geothermal development has become very important and we have to survey new prosrects and the relationship between existing hot springs and prospective geothermal fields for power generation. Then, we carried out a project with the Ministry of Environment, Japan, Development of an advanced geothermal reservoir management system for the harmonious utilization with hot spring resources near Hachijo island geothermal power plant and Minami-Izu geothermal field at the south edge of Izu peninsula. We carried out the tracer test in September, 2012 at Minami-Izu geothermal field to survey the flow properties (direction, rate etc.) in existing hot spring reservoir including fault and the relationship with deeper reservoir. Site of Minami-Izu Hot Spring Region The Minami-Izu geothermal field exists on the Izu peninsula at the south end of the Shizuoka prefecture, Japan as shown in Figure 1. This geothermal field has several hot spring wells with temperatures of about 100 degree C at a depth of around 150 m. This Minami-Izu hot spring area is about 2km E-W and 1km N-S wide and in this region, many hot spring wells exist as shown in Figure 2. At the west area of this field, Kano hot spring area, hot spring fluid is produced from about 150 meter depth and the Cl concentration is about 10,000 mg/l. The Cl concentration Figure 1. Location of the Minami-Izu area. 879

2 Injection K-3 Monitoring Using well Used well Kano Area K-6 r=300m r=200m K m gradually decreased at east area, Shimogamo hot spring area (TEP- SCO, 2011). Then, the up-flow region of the hot spring fluid was thought as existing in Kano hot spring region and the flow direction of hot spring fluid from west (Kano) to east (Shimogamo). A monitoring well was drilled to 600 meter depth in the Kano area from January to February Secondly, the injection well was drilled to 150 meter depth, the same feed point of many hot spring wells, in August 2012 about 70 meter north from monitoring well. The injection of hot spring fluid from the monitoring well to the injection well was carried out from 12 September to 26 September, Method of Tracer Test K-11 Figure 2. Map of hot springs in Minami-Izu area. For this tracer test sodium fluorescein ( uranine dye ) was used. The reason of using uranine are as follows; (1) safety for bathing and human health in hot spring, (2) easy to real-time monitoring using optical fiber fluorometer system (Matsunaga et al.,2001, Yanagisawa et al.,2002, Yanagisawa et al.,2003, Yanagisawa et al., 2009), (3) easy onsite laboratory analysis by using a fluorescent spectrophotometer, (4) no decomposition at about 100 degree C estimated on hot spring fluid. Then, about 500g sodium fluorescein tracer dissolved in 200L water was injected into the injection well from 8:59 to 9:02 on 16 September m 200m 300m 400m 500m 600m Monitoring Well Pump Casing until 350meter depth (1) fracture Tracer Injection Injection Well Tracer Flow Shimogamo Area Hot spring wells (Tracer monitoring) (K-13) (K-3) (K-11) 700m Figure 3. The schematic diagram of tracer test; (1)moving to deeper, (2) flowing horizontal layer, (3) moving to upper, (4) escape. (4) (2) (3) (K-6) The schematic diagram of this tracer test is shown in Figure 3. Tracer was injected to the Injection well and after tracer reached the bottom of the Injection well, tracer flow was separated several ways; (1) moving to deeper layer and be found at monitoring well, (2) flowing in shallow horizontal layer and found in existing hot spring wells, (3) moving to upper layer, (4) escape in formation (parasitic loss). We monitored the tracer appearance at 5 hot spring wells, Daigaku-yu (K-13), Kyodou-yu (K-3), Tamagawa-yu (K-11), Yaegase-yu (K-6) and the Monitoring well as shown in Figure 4. In this area, a fault was found (Sameshima and Iwahashi(1970) ; Yamada(1977)) ending to the NNW-SSE direction dipping 80 degrees to the east near Injection well and Daigaku-yu (K-13). From Injection well, the Monitoring well exists 70 meter south, the Daigaku-yu (K-13) exists 70 meter south-east, the Kyodou-yu (K-3) exists 100 meter east, the Tamagawa-yu (K-11) exists 170 meter ENE and Yaegase-yu (K-6) exists 250 meter north-east. And all tracer-monitoring well exists along the Nijo river from SW to NE (as shown in Fig.1). Nijo River Injection Well Monitoring Well Fault Sameshima and Iwahashi(1970) Yamada (1977) Daigaku (K-13) Figure 4. Sampling point of tracer test and existing fault. Yaegase (K-6) Kyodou (K-3) Tamagawa (K-11) 100m For Daigaku-yu (K-13), Kyodou-yu (K-3) and Tamagawa-yu (K-11), the fiber-optic fluorometer was used to obtain real-time fluorescence counts as shown in Figure 5 (Matsunaga et al.,2001, Yanagisawa et al.,2002, Yanagisawa et al.,2003, Yanagisawa et al., 2009). Tracer concentrations at were monitored from 16 September 2012 for more than 3 months. Fluid samples were taken from 5 wells for tracer analysis in laboratory. From monitoring well, we collected sample 3 times per day from 13 September to 26 September. For other 4 wells, we collected fluid samples 3 times per day in September and 2 times per day after October. Fluorescein concentration of bottled sample was measured by a fluorescent spectrophotometer of FP made by JASCO in laboratory. 880

3 Results of the Daigaku-yu (K-13) Near Fault Figure 7 shows the tracer response of Daigaku-yu (K-13) detected by optical fiber system. Tracer firstly appeared at 18:00 on 16 September, only 9 hour after tracer injection. The tracer concentration rapidly increased and reached peak concentration on 19 September, 3 days after injection. The tracer concentration reached 50 ppb on 19 September, corresponding to an intensity count of 1,000 by optical signal. The flow rate from injection well to Daigaku-yu at peak concentration is estimated to about 0.3mm/sec. Figure 5. Diagram of optical fiber system. Results and Discussion Result at the Monitoring Well Figure 6 shows the tracer response at the Monitoring well until 27 September. From 21 September, 5 days after injection, tracer seems to appear and the concentration gradually increased and reached 0.5ppb at 27 September. Due to pumping finished at 27 September, we cannot measure tracer concentration after that. But this result suggests the existence of fluid flow from about 150 meter depth to deeper reservoir. Figure 6. Tracer response at the Monitoring well. Figure 7. Tracer response of Daigaku-yu (K-13) by optical fiber system. Figure 8. The tracer return ratio at Daigaku-yu (K-13). This tracer response seems to be similar as many fractured reservoir of EGS field. At Hijiori, EGS test site, injected tracer returned only several hours. (Matsunaga et al., 2001, Yanagisawa et al.,2002). And at EGS field, tracer return ratio was relatively high, for example 50 % at 2 production well of Hijiori and about 78% at Habanero site, Cooper-basin, Australia (Yanagisawa et al., 2009). Then, we estimated the tracer return ratio of Daigaku-yu (K-13) from tracer response curve, the mean fluid production rate (about 200L/min) and injected tracer mass. The change of tracer return ratio at Daigaku-yu (K-13) is shown as Figure 8. The tracer return ratio reached about 7 % at tracer peak on 19 September. And the tracer return ratio gradually increased and reached about 30% at end of November. This ratio is about half of the two production wells of Hijiori. Then, the fractured system seem to exist between injection well and Daigaku-yu. From previous geological survey, a fault exist in this area as shown in black dash line of Figure 4 (Sameshima and Iwahashi, 1970, Yamada, 1977) and the fault is in NNW-SSE direction, almost vertical but dipping 80 degree to the east. Figure 4 shows the fault exists on a trend that intersects both injection well and Daigaku-yu (K-13), and the injected tracer flowed along the fault to the SE and appeared at Daigaku-yu (K-13). Results of the Hot Springs at East of Injection Well We monitored tracer response of Kyodou-yu (K-3) and Tamagawa-yu (K-11) using optical fiber system with Daigakuyu (K-3), but we cannot detect appearance tracer due to very low 881

4 concentration less than 0.05ppb corresponding to 1 count of the detector signal. Then we continue on site measurement by a fluorescent spectrophotometer using bottled sample from wells until 19 December Firstly, Figure 9 shows the tracer response at Kyodou-yu (K-3). Tracer first appeared on 5 October, 20 days after injection. The tracer concentration increased gradually and reached 0.035ppb on 19 December. From this response curve, the tracer concentration did not reach a peak. And the concentration at 19 December is about 1/2000 of the peak concentration of Daigaku-yu. Figure 10 shows the tracer response at Tamagawa-yu (K-11). Tracer firstly appeared on 27 September, 11 days after injection. Figure 9. Tracer response at Kyodou-yu (K-3). Figure 10. Tracer response at Tamagawa-yu (K-11). Figure 11. Tracer response at Yaegase-yu (K-6). The tracer concentration increased gradually and reached peak concentration of 0.24ppb on 26 October, 40 days after injection. The appearance of tracer at Tamagawa-yu was earlier than Kyodou-yu, closer to the injection well. The peak tracer concentration was about 8 times than Kyodou-yu and about 1/200 of the peak concentration of Daigaku-yu. The flow rate from injection well to Daigaku-yu at peak concentration is estimated to about 0.05mm/sec. Figure 11 shows the tracer response at Yaegase-yu (K-6). Tracer firstly appeared on 20 October, 34 days after injection, and the tracer concentration increased gradually to 0.029ppb on 19 December. From this response curve, the tracer concentration did not reach to peak similar as Kyodou-yu. From the tracer response of these wells, we recognized the tracer flow in hot spring reservoir from injection well to east along the Nijo River. The return of tracer at Tamagawa-yu (K-11) was earlier and tracer concentration was higher than Kyodou-yu (K-3) despite of distance from injection well. Then we estimate the relationship the main flow path and fault. Figure 4 shows the direction of the proposed flow path at right angle to the NW-SE fault as orange dashed line. From injection well, the orange line directly intersects at Tamagawa-yu (K-11). Then the direction of main flow in hot spring reservoir seems to be the right angle of the fault, ENE direction, and gradually spread to surrounding wells. The trend of the direction of groundwater flow in this area toward the east is corresponds to the estimated flow direction identified by SP survey in this project (Yasukawa et al., 2013). Summary The tracer test is carried out in high temperature hot spring fluid at Minami-Izu geothermal field, Shizuoka, Japan. From the results of the tracer test, we conclude that; 1) At monitoring well, tracer appeared 5 days after injection and this suggests flow path to deeper reservoir. 2) At Daigaku-yu (K-13) well, the first tracer appeared 9 hours after tracer injection and tracer concentration rapidly increased and showed peak at three days after injection. The return ration at K-13 is estimated about 30%. 3) In other wells, at Tamagawa-yu (K- 11) about 150 meter from injection well, the first tracer appear at 10 days after injection and earlier than Kyodou-yu (K-3). 882

5 4) The main flow injected tracer is along with the large fault between the injection well and K-13 well and the main flow in hot spring reservoir appears to be associated with a newly identified flow path in the E-NE direction. Acknowledgement This study is conducted under a project Development of an advanced geothermal reservoir management system for the harmonious utilization with hot spring resources sponsored by the Ministry of Environment, Japan. References Matsunaga, I. Sugita. H., and Tao, H., Tracer Monitoring a Fiber-Optic Fluorometer During a Long-Term Circulation Test at the Hijiori HDR Site. Proceedings of 26th Stanford Workshop on Geothermal Reservoir Engineering, p Sameshima, T. and Iwahashi, T., Geothermal Structure of Shimogamo Hot Spring on the Izu Peninsula. Geoscience Reports of Shizuoka University, 2-(1), (in Japanese with English summary) TEPSCO, Assessment report on MIDORINO-BUNKEN KAIKAKU project at Minami-Izu town. pp. 365 (In Japanese). Yamada, E., Stratigraphy and Geological Structure of the Neogene Formations, Southeastern Part of the Izu Peninsula, Japan. GSJ Bulletin, 28, Yanagisawa, N., Matsunaga, I., Sugita, H. and Tao, H., Reservoir Monitoring by Tracer Test of a 2001 Long Term Circulation Test at the Hijiori HDR site, Yamagata, Japan. Geothermal Resources Council Transactions, 26, p Yanagisawa, N., Matsunaga, I., Sugita, H. and Tao, H., Reservoir Monitoring by Tracer Test of a 2002 Dual Circulation Test at the Hijiori HDR site, Yamagata, Japan. Geothermal Resources Council Transactions, 27, p Yanagisawa, N., Rose, P. and Wyborn, D., First tracer test at Cooperbasin, Australia HDR reservoir. Geothermal Resources Council Transactions, 33, p Yasukawa, K., Ishido, T., Matsubayashi, O. Uchida, T., Sakaguchi, K. and Matsuyama, K., Interpretation of MT and SP survey results at Minami-Izu geothermal field, Japan. Proceedings of 38th Stanford Workshop on Geothermal Reservoir Engineering, p

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