Drilling Make-Up Well (O-21) at Otake Geothermal Power Station To Maintain Power Output and Reservoir Stability

Transcription

1 日本地熱学会誌第 37 巻第 2 号 (2015) 53 頁 57 頁短報 J.Geotherm. Res. Soc. Japan Vol. 37. No. 2(2015) P.53 P.57 Drilling Make-Up Well (O-21) at Otake Geothermal Power Station To Maintain Power Output and Reservoir Stability Yuki FUJII *, Hiroki SAITO *, Yumi KIYOTA *, Tetsuya YAHARA * and Toru DAIBO ** (Presented 31 July 2014, Received 26 December 2014, Accepted 14 April 2015) Abstract The Otake Geothermal Power Station (12.5MW) is located in Otake-Hatchobaru geothermal field, one of the most active geothermal fields in Japan. This power station has been operating since 1967 and maintaining a stable operation since 1988 using 4 production wells. However, the electric power output has declined since 2006 due to the decreasing mass flow of the main production well (O-22). In 2010, a magnetotelluric (MT) survey was conducted to detect a drilling target for a make-up production well. In 2012, make-up well O-21 was drilled to the target detected in the MT survey and it successfully discharged steam and water. As a result of the successful drilling of the make-up well, the power output of the Otake Geothermal Power Station recovered to its 2006 level of output. Keywords: geothermal, the Otake Geothermal Power Station, stable operation, make-up well 1. INTRODUCTION The Otake Geothermal Power Station (12.5MW) is located in a volcanic area of central Kyushu, Japan. The power station has been operated by Kyushu Electric Power Co., Inc. since The power generation method is a single-flash type. The Hatchobaru Geothermal Power Station (110MW), the largest geothermal power station in Japan, is located about 2km south of the Otake PS (Figure 1). The Otake PS had been generating a stable electric power output using 4 production wells until From 2006, a power decline was experienced. The main factor in the decline was the decreasing mass flow of the main production well (O- 22), which was caused by scale deposition in the well (Figure 2). Thus O-22 could continue to be used only with repeated dredging work [1]. In 2010, a magnetotelluric (MT) survey was carried out to detect a drilling target for a make-up well. In 2012, makeup well O-21 was drilled targeting the fault structure detected in the MT survey and it penetrated some permeable structures. In the production test, O-21 successfully discharged steam and water. The installation of O-21 allowed the power output of the Otake PS to recover to its 2006 level of output. This paper considers the detection of the drilling target by MT survey, the results of well drilling and the production test of O-21. * ** West Japan Engineering Consultants, Inc. (West JEC) Kyushu Electric Power Company, Inc. (KEPCO) The Geothermal Research Society of Japan,

2 Figure 3. Fault distribution in the Otake area [4] Figure 1. The location of Otake PS & Hatchobaru PS. The geothermal fluid is estimated to be up-flowing and stored along the permeable zone related to the fault systems. The fluids supplied to the Otake PS are stored from depths of between 300m and 600m (elevation of 700m to 400m asl.). The geothermal reservoir in the Otake area is considered to be of the mushroom-shaped type (Figure 4) [3]. Figure 2. Steam flow rate and well head pressure of O-22 [1] 2. GEOTHERMAL STRUCTURE The Otake-Hatchobaru geothermal field is located at the southern margin of Shishimuta sedimentation zone. The Otake PS is surrounded by volcanoes such as Mt.Sensui, Mt.Kuroiwa and Mt.Waita. Moreover, there are some hot springs such as Sujiyu hot spring, Hizenyu hot spring and Otake Jigoku in the vicinity of Otake PS. Therefore Otake- Hatchobaru Geothermal field, where the Otake PS is located, is one of the most active geothermal fields in Japan. At the Otake geothermal field, the southern area is the production area, and the northern area is the reinjection area. The heat source of this field is estimated to be volcanic activity related with Mt.Kuroiwa located about 2.5km southeast of the Otake PS [2]. The geological structure in the Otake geothermal field is characterized by the presence of NW-SE trending fault systems. The main NW-SE trending faults are the Sujiyu Fault, the Hizenyu Fault, the Yokoo Fault and the Otake Fault (Figure 3). Figure 4. Conceptual model of Otake area [4] Altered zones, which constitute an impermeable layer, are distributed in the zone from 200m to 500m above the geothermal reservoir in Otake geothermal field. These altered zones play the role of the cap rock in the Otake geothermal system. The temperature of the geothermal reservoir (or 54

3 fluids) in Otake geothermal field is estimated to be o C, based on the well characteristics test. The homogenization temperatures of fluid inclusions are also in the same range [5]. 3. MT SURVEY In 2010, an MT survey applying 3D inversion analyses was carried out to detect a drilling target for the make-up well. The size of survey area was about 2 x 2 km. There were 47 stations spaced at intervals of about 300m. The survey revealed that a low resistivity zone is distributed at a depth of 150m - 300m in the eastern part of the Otake field. The low resistivity zone was considered to result from argillitic rock containing hydrothermally altered minerals. Furthermore, 3 resistivity discontinuities were detected in the deep part of this zone (Figure 5). Resistivity discontinuities R1 and R2 were found to correspond to fault systems detected earlier by gravity survey and an Acoustic Emission hypocenter distribution study, while resistivity discontinuity R3 was a newly detected fault indicator. Moreover, It was estimated that the resistivity structure at relatively great depth of R1 and R3 reflect the presence of a hydrothermally altered zone formed under high temperature conditions. Therefore R1 and R3 were concluded to be indicators of a highly permeable zone controlling geothermal fluid flow [6]. Figure 5. Resistivity discontinuities detected in MT survey [6] 4. THE ASSESSMENT OF O-21 Make-up well O-21 was planned to penetrate both resistivity discontinuities R1 and R3, which were estimated to be the path of high temperature geothermal fluid. Production testing of O-21 in 2012 following well drilling indicated that O-21 penetrated some permeable structures at relatively great depth and successfully discharged steam and water. In the course of evaluation O-21, cutting observations, X-ray diffraction analyses, measurement of homogenization temperature of fluid inclusions, static well logging, injection well logging and fall-off testing were carried out. At the production test stage, a well characteristics test using the orifice method, production well logging and geochemical analyses of discharged fluid were conducted. As a result of these studies, the main feed zone of O-21 was recognized to lie at a depth of 1,976m (measured depth) and the discharged mass flow was equivalent to around a 3,000kW electric power output (Table 1, Figure 6). The alteration level at the main feed zone of O-21 was also recognized to be similar to that of O-22 which is near O-21. Geochemical analyses of discharged fluid from O-21 also showed that it was similar to that from O-22. The relationships between the Cl concentration of reservoir fluid and the specific enthalpy based on silica temperature are particular example (Figure 7). Therefore the reservoir condition for the discharged fluid of O-21 was recognized to be similar to that of O-22. Thus, the main feed zone of O-21 was presumed to connect with that of O-22 by the structure which was indicated by resistivity discontinuity R3. But the temperature of the geothermal reservoir (or fluids) at O-21 was higher than that of O-22. It is estimated that the difference of temperature is caused by the degree of reheating for injected fluid. With the installation of O-21, the power output of Otake PS has recovered to its 2006 level and is expected to maintain stable power output from now on. The long term utilization of O-21 may change the discharged state of O-21 and O-22. At present, the significant interference between O-21 and other wells or gravity variation near O-21 have not been recognized. But the decreasing mass flow of O-22 was caused by scale deposition in the well, so the potential of O-21 also should be realized by taking care of scale deposition in O-21 itself and be used carefully with monitoring of the discharged state, the geochemical analyses and the gravity variations. 55

4 Table 1. The productivity of O-21 Well head pressure (MPaG) Steam (t/h) Water (t/h) Enthalpy (kj/kg) 1,619 1,617 1,571 Power output (kw) 3,600 3,400 3, CONCLUSION In 2010, a prospective drilling target for a make-up well was detected by an MT survey which was carried out to restore the declining power output of the Otake PS. Make-up well O-21 was drilled in 2012, penetrating some permeable structures and successfully discharging steam and water, with a steam flow equivalent to around 3,000kW of electric output. The power output of Otake PS has now recovered to its 2006 level and is expected to maintain stable power output from now on with careful monitoring of various kinds to sustain the productivity of O-21. Figure 6. The result of O-21 drilled in Figure 8. Production test of O-21 ACKNOWLEDGMENT We wish to thank the members of Kyushu Electric Power Company, Inc. for their helpful advice and cooperation in this study. Figure 7. Specific enthalpy vs Cl concentration REFERENCES [1] Wada et al., The mechanism of productivity decline in Otake production well O-22, 2009 Annual Meeting Geothermal Research Society of Japan Abstracts, 2009, B19. [2] Yahara et al., Geothermal Fluid Flow System in the Otake 56

5 Field, 1999 Annual Meeting Geothermal Research Society of Japan Abstracts, 1999, B45. [3] Taguchi et al., Subsurface Thermal Structure Revealed by Fluid Inclusion Thermometer at the Otake Geothermal Field, Kyushu, Journal of the Geothermal Research Society of Japan, 7(4), 1985, pp [4] Baba et al., Geothermal Well Drilling in the Otake-Hatchobaru Geothermal Field. JGEA, 28(4), [5] Matsumoto et al., Operation Record and Management of the Otake and Hatchobaru Geothermal Power Station, Chinetsu, Japan Geothermal Energy Assoc, 26(4), 1989, pp [6] Saito et al., Geothermal Structure in the Southeastern Area of Otake Geothermal Field derived from Magnetotelluric Survey, 2011 Annual Meeting Geothermal Research Society of Japan Abstracts, 2011, A30. 短報大岳地熱発電所における補充蒸気井 (O-21) 掘削出力と貯留層の安定維持を目指して藤井勇樹齋藤博樹清田由美矢原哲也大保徹概要九州電力大岳発電所は日本で最も活発な地熱地帯の 1 つである大岳 - 八丁原地熱地帯に位置する本発電所は 1967 年に運開され,1988 年以降は 4 本の蒸気井のみで比較的安定的に運転されていたしかし 2006 年頃から, 主力蒸気井である O-22 のスケール析出によって出力は減衰傾向を示すようになったそのため 2010 年に, 補充井の掘削ターゲットの選定を目的とした MT 探査が実施された 2012 年にそのターゲットに補充蒸気井掘削 (O-21) が行われ, 蒸気熱水の噴出に成功したことで, 現在大岳地熱発電所の出力は回復に転じている貯留層及び出力の安定維持のため各種モニタリング等を継続し適切な運用を行っていくことが求められるキーワード : 地熱, 大岳発電所, 安定運転, 補充井 57