3D Numerical model of the Obama hydrothermal geothermal system, Southwestern Japan

Transcription

1 3D Numerical model of the Obama hydrothermal geothermal system, Southwestern Japan Hakim Saibi Computational Geosciences Modeling, Simulation and Data Analysis ISSN Volume 15 Number 4 Comput Geosci (2011) 15: DOI /s

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3 Comput Geosci (2011) 15: DOI /s ORIGINAL PAPER 3D Numerical model of the Obama hydrothermal geothermal system, Southwestern Japan Hakim Saibi Received: 11 June 2010 / Accepted: 18 April 2011 / Published online: 6 May 2011 Springer Science+Business Media B.V Abstract The Obama geothermal field is one of the most interesting geothermal areas in Kyushu Island, Southwestern Japan, because of its large number of high-temperature springs. A 3D numerical simulation study using the simulator TOUGH2 (module EOS3) was carried out to obtain a comprehensive hydrothermal model of the field. From previous geochemical studies, two main fluid sources were suggested for the Obama geothermal system: cold (sea, surface, and ground) water and deep geothermal fluids. We propose two heat sources, a lateral one at the eastern boundary of the system, near the West Unzen High Temperature Body located west of the Unzen fumarolic field, and a second one beneath the Obama geothermal field. The first source contributes the system by 150 C fluids. The second source contributes by 100 C fluids. Our model indicates that the first source has a temperature of 150 C, which agrees with the results from previous geochemical studies. The low enthalpy of the second source could be explained by the mixing of geothermal fluids with seawater, as the area is near the seashore and is highly faulted. The model that was developed can explain many of the subsurface processes active in the Obama geothermal field. H. Saibi (B) Laboratory of Exploration Geophysics, Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka , Japan saibi-hakim@mine.kyushu-u.ac.jp, saibi.hakim@gmail.com Keywords Numerical model Obama geothermal field Heat source Japan 1 Introduction The Obama geothermal field is located in Southwestern Japan, in the western part of Kyushu Island, on the Shimabara Peninsula, west of the Unzen volcano, and facing Chijiwa Bay (Fig. 1). The origin of the field, part of a volcanic-related hydrothermal system, may be linked to emanations from a magmatic reservoir located beneath Chijiwa Bay at 15 km in depth [4]. We will present a hydrothermal model of the Obama geothermal field that was developed using the simulator TOUGH2 [12] that solves numerically the mass and energy balance equations. TOUGH2 [11], based on the earlier MULKOM [10], is a general-purpose reservoir simulator capable of simulating non-isothermal flows of multi-component, multi-phase fluids in porous and fractured media. Its finite volume formulation gives it the flexibility to model 1D, 2D, or 3D problems with irregular computational grids [1]. We simulated the hydrothermal geothermal system at Obama area in order to understand its natural state. The annual average atmospheric temperature of 16.7 C and the atmospheric pressure of bars were assigned to the ground surface (i.e., the top of the model), which was assumed to be permeable. To obtain the most suitable model, we compared and matched the calculated temperature against the temperature profiles of wells UZ-2 and UZ-4. The hydrothermal model of the Obama geothermal system presented below agrees with previous models

4 710 Comput Geosci (2011) 15: Fig. 1 a Location of the Obama Geothermal Field, Kyushu Island, Japan. The black squares show the location of geothermal areas relatedtoquaternary-recent volcanic activity. Active volcanoes are also shown. b Location of the Obama Geothermal Field in Shimabara Peninsula that were based mainly on interpretation of geochemical data [7, 9, 16, 19, 20]. Actually, the Obama geothermal field does not have geothermal power plant or other kinds of exploitation of the hot fluids. The results of this numerical model will surely help the managers to understand the geothermal potential at Obama and the limitation of production of the Obama hot fluids for a good exploitation of the field in the future. 2 Brief history of Obama geothermal field The Obama geothermal field is one of the most promising geothermal fields in Japan. The Obama Spa is historically noted for the abundance and high temperature of its hot spring waters. The heat discharged by the spring waters (23.72 J/s) is the highest in the country, which exceeding even the J/s reported for the Beppu Spa [6]. Historically, the hot water flowing out on the seashore during ebb periods was used for hotspring cures. The water being discharged is classified as being of hyper-thermal NaCl type. The location of the geothermal field so close to the coast allows the incorporation of seawater into the circulating fluid regime [14]. Salt (NaCl) production played an important role in the history of the Obama geothermal field. Production through evaporation powered by the heat of the geothermal fluids began in The number of salt factories increased rapidly to 80 in 1951, and the total extraction of geothermal water was about 50,000 m 3 /day. In 1948, salt output was 7,600 t, and from 1952 to 1956, 10,000 t/year or more.

5 Comput Geosci (2011) 15: The Obama Spa prospered economically; however, over-pumping of the geothermal aquifer and seawater intrusion increased the water salinity and decreased water temperatures [19], which stopped boiling in the wells. In 1950, the loss of the boiling wells became such a large problem that the hot springs were in danger of becoming unusable. In 1955, the number of extraction wells was reduced from about 40 to 8, and the salinity of the water increased to 25,000 mg/l, which was three to four times the value before beginning salt production. The extraction of geothermal water was lowered from 50,000 to 32,000 m 3 /day, although salt production did not decrease. On 17 September 1959, Typhoon 14 hit the Obama Spa and caused serious damage. It was so big that 32 salt factories were abandoned by the end of that year. Subsequently, only 24 wells were left for use in public baths, hotels, and inns; hot water production was reduced to 8,150 m 3 /day. As a result, the geothermal system gradually recovered; i.e., the number of boiling wells increased, water temperatures rose, and water salinity decreased. At present, the conditions of the thermal springs are approaching those before Microgravity monitoring surveys were conducted over the Obama field from 2003 to They showed an increase of about 95 μgal between the surveys of 9 April 2003 and 30 March 2004, which was attributed to rising groundwater levels and higher reservoir pressures [17]. The mass gain in the system was estimated to be about 15 Mt [18]. The geothermal reservoir pressure recovery is still continuing. studies in the Obama area indicate the existence of faults striking mainly in the E W direction [6, 8, 16]. However, there are also N S, NE SW, and NW SE trending faults [16]. The lithologic column for the Obama area, developed on the basis of petrographic and macroscopic analyses of drill cuttings and core samples [6], is composed mostly of Quaternary and Pliocene volcanic formations (Fig. 2). For numerical modeling purposes, the column was subdivided into three types of materials. Layer 1 includes the Takadake and Obama volcanic rocks (mainly hornblende lavas and tuff breccias) and the lower part of the Tatsuishi Formation (volcanic tuffs and silts). In our model, these lower layers allow the lateral flow of fluids. Layer 2 corresponds to the upper parts of that Tatsuishi Formation (volcanic tuffs and silts). Layer 3 represents the Kuchinotsu Group (volcanic and sedimentary rocks), which is considered to be the permeable basement for the Obama geothermal reservoir (Fig. 3). 3 Geology and tectonic setting The Obama geothermal field is located in the southwestern part of the large Beppu-Shimabara Graben (Fig. 1), where a number of geothermal springs discharge along fault traces. The graben lies at the junction of the Southwest Japan and Ryukyu Arcs and was formed either by extension associated with the subduction of the Philippine Sea Plate beneath the Eurasian Plate [3] or by right-lateral shear caused by oblique subduction of the Philippine Sea Plate beneath Kyushu Island [21]. Within the Beppu-Shimabara Graben, there are many Quaternary volcanoes, some of which are active (i.e., Yufu, Tsurumi, Garan, Kuju, Aso, and Unzen, from east to west) with associated geothermal systems. The Unzen Graben is in a marginal zone of depression that is affected by regional and local stress fields. Extension is in the N S direction and compression in the E W direction [5]. Geological and geophysical Fig. 2 Geologic map of the Obama geothermal field (modified from NEDO [6])

6 712 Comput Geosci (2011) 15: Fig. 3 Stratigraphic log of UZ-2 (modified from NEDO [6]). The lateral flow occurs in the lower part of layer 1 (Obama volcanic rock) which corresponds to Material 8 in the numerical model 4 Conceptual model During the period, we have carried out gravity measurements in the area west of the Heisei- Shinzan lava dome that was formed during the eruption of the Unzen volcano [19]. Measured gravity changes are attributed to shallow groundwater level variations. In the lower regions of the Unzen volcano, groundwater movement follows the topography [19]. The downflowing colder groundwaters may mix with hotter geothermal fluids and discharge at the Obama hot springs. We performed a hydrogeochemical study of the Obama hot spring waters, which included chemical geothermometry and evaluation of fluid mixing models [15]. The results show that (1) all the hot spring waters are of NaCl type, (2) the estimated reservoir temperature is between 150 C and 200 C, and (3) the Obama geothermal reservoir is recharged by deep, hot chloride waters and by colder meteoric/surface and seawaters, with a 24% seawater mixing ratio [19]. Ohsawa [7] stated that the Obama hot springs are part of a liquid-dominated geothermal system with waters of NaCl type. The system is being recharged mainly by the hot fluids from a high-pressure magma reservoir at depth. At shallower levels, the ascending thermal waters migrate laterally toward the western coastal regions of the Shimabara Peninsula and conductively cool. Finally, they mix with the colder surface and ground waters to form the boiling or hot (about 200 C) waters of the Obama hot spring [7]. A large-scale hydrothermal model for the region beneath Unzen volcano was presented by Fujimitsu et al. [2]. Their model succeeded in reproducing the four large geothermal systems on the Shimabara Peninsula, which from west to east are: Obama hot springs, West Unzen High Temperature Body (WUHTB), Unzen fumarole field, and Shimabara hot springs (Fig. 4). In their simulation study, Fujimitsu et al. [2] included three fluid pressure sources (A, B, and C) inferred from geodetic data gathered during the eruption. These sources are, respectively, beneath the Unzen volcano, the Unzen fumarole field, and the WUHTB. Source C, below the WUHTB, is considered to be a magma reservoir at about 8 km depth. Figure 4 depicts a W E cross section of the conceptual model developed by Saibi and Ehara [19] (especially the Obama area), showing the assumed flowpaths for the different types of waters of the geothermal system. At the western margin of the Obama reservoir, at or near Chijiwa Bay, cold water (including seawater) may be entering the system. At its eastern edge, the reservoir is recharged by thermal waters from the WUHTB and/or the Unzen fumarole field. Through its bottom, the geothermal aquifer may be recharged by thermal conduction and/or by rising of hot waters flowing through fractures and faults. The model shows that the magmatic fluids/emanations originate from

7 Comput Geosci (2011) 15: Fig. 4 Conceptual geothermal model of Obama geothermal fluid deduced from geochemical and numerical analysis. The Obama geothermal field is a liquid-dominated geothermal system. The deep chloride fluids travel nearly vertically to the surface and then mixed with lateral flows. Note that the seaside location implies the intrusion of seawater, which is also chloride-rich Fig. 5 Plan view of the study area and the computational grid. Triangles represent the location of wells used to estimate aquifer properties as well as fitting the simulation results. For the fitting of the simulation results, we used thedataofuz-2anduz-4. The T-3 and the New Drill were not considered in this study because they are not deep wells

8 714 Comput Geosci (2011) 15: a main magma chamber located under Chijiwa Bay. However, magmatic fluids could also be derived from separate shallower chambers beneath the WUHTB and/or Unzen fumarole field, which may be fed by the main magma chamber located deeper and to the west (Fig. 4). 5 Modeling domain Fig. 6 Characteristics of the most top layer in the numerical model A detail of the computational grid used to model the Obama geothermal field is shown in Fig. 5. The model covers 49 km 2 (slightly larger than the study area), extending 7 km in the east west direction and 7 km in the north south direction. The mesh is rather coarse with 26 grid blocks in the east west direction (i index) and 37 blocks in the north south direction ( j index). It is finer around the Obama well field. Horizontal grid block dimensions vary between 0.1 and 1.6 km in the E W direction and between 0.1 and 1.2 km in the Fig. 7 a, b Location of the lateral and the bottom heat sources in the numerical model

9 Comput Geosci (2011) 15: Table 1 Permeabilities and bulk properties of the different used materials in the simulation model Material 1 Material 2 Material 3 Material 4 Material 5 Material 6 Material 7 Material 8 (Fracture (Volcanic (Tasuishi (Kuchinotsu (Cold (Air) (Heat (Low zone) rocks) formation) unit) seawater) source) permeability zone) located at the second portion of Material 2 Porosity [ ] Permeability 1.5E E E E E E E E-13 in X [m 2 ] Permeability 1.5E E E E E E E E-16 in Y [m 2 ] Permeability 3.0E E E E E E E E-16 in Z [m 2 ] Thermal conductivity [W m 1 K 1 ] Specific heat capacity [J kg 1 K 1 ] Density [kg m 3 ] N S direction. Vertically, the model extends from the ground surface (i.e., between sea level elevation and 800 m above it) to 1,000 m below sea elevation. A total of 19 layers (k index), each 100 m thick, were used. The uppermost grid blocks located above the ground surface are left void ; the total number of non-void blocks in model is 11,025; their total volume is km 3. Two thin film layers with infinite volume were added to the model: the first one is to represent the sea cold water especially in the left side of the model, and the second one is to represent the air especially in the right side of the model (Fig. 6). To conduct this kind of simulation, we used the module EOS3 which can handle water, air, and thermal fluids. Fig. 8 Geological structure of the reservoir model in vertical planes (x z) passing through the well field and fault structure (left side) with the simulated temperature distribution after 1,000,000 years (right side)

10 716 Comput Geosci (2011) 15: Fig. 9 Geological structure of the reservoir model in horizontal planes (x y) at different levels (left side) with the simulated temperature distribution after 1,000,000 years (right side). The plans k = 11 and k = 12 show the location of the lateral heat source

11 Comput Geosci (2011) 15: Table 2 Characteristics of the two heat sources Heat sources Temperature Pressure Area of ( C) (bars) inflow Lateral heat source km 2 Bottom heat source km 2 For more details on the governing equations used in TOUGH2, please refer to Pruess [13] paper. 6 Results and discussion 6.1 Natural state The fluid pressure distribution in the grid was assumed to be hydrostatic; the ground surface boundary pressure was set at bars. The lateral boundaries (north, west, and south) were treated as being closed to heat and mass flow, but the eastern boundary was opened in local parts to include the lateral heat source (Fig. 7a). During the calculations, both temperature and pressure were fixed at all the boundaries except the eastern and the bottom boundaries where heat source inflow exists (Fig. 7b). The simulation was run for 1000,000 years, so that the temperature and pressure distributions in the model did not change significantly with time. At the point, they were assumed to represent natural-state (pre-exploitation) conditions in the Obama geothermal system. As mentioned in Section 3, eight types of materials were distributed throughout the computational grid reproducing the physical characteristics of the Obama system. We changed the permeability of these materials until we got a reasonable fit between the measured and computer temperature profiles in wells UZ-2 and UZ-4. Unfortunately, we do not have pressure data from geothermal wells in Obama Geothermal Field. The only parameter for comparison is temperature. In this study, anisotropic permeabilities were assumed. For the Material 1 (fractures), we assumed high permeability in the Z-direction. However, for the Materials 2, 3, 4, and 8, we assumed high permeabilities in the X direction. The best match was obtained when the permeability of Material 2 was between 100 and 1,000 times larger than that of Material 4 and between 1 and 10,000 times that of Material 3 (Table 1). Rock properties other than permeability (i.e., porosity, thermal conductivity, specific heat, and density) Fig. 10 Flow patterns in the different plans x y, x z ( j = 25)andy z (i = 16) after 1,000,000 years simulation. An upflow of hot waters occurs at the Obama geothermal field and also a lateral flow occurs from the eastern part of the Obama geothermal field. The downflow of the groundwater occurs especially at elevated areas

12 718 Comput Geosci (2011) 15: used as input to the model were based on measurements made on each rock type [6]. Figure 8 shows how the three major formations were assigned in the east west section at j = 16, j = 17, j = 18, j = 19, and j = 26. A vertical fracture zone was added to sections located at j = 16, j = 18 (Fig. 8), and j = 19; thelocation of this zone is based on geological information. Material 8 with low permeability was added to the model in order to achieve a good matching between the measured and simulated temperatures. Figure 9 shows how the three geological formations were assigned in the horizontal plans at k = 9, k = 11, k = 12, k = 14, andk = 19. We can see how the inflow of the hot fluids flows through the system and the fault structure. The permeability values and the bulk properties of the materials used in the simulation model are summarized in Table 1. From the temperature profiles of UZ-4 and UZ-2, a temperature anomaly is observed at depth between 200 and 600 m for UZ-4 and 100 to 300 m for UZ- 2. This anomaly could be explained by a lateral flow from the eastern part of the geothermal field. We have set two heat sources: the first one is located laterally at the eastern side of the model, which is located beneath the WUHTB. The second one is under the Obama geothermal field, where the hot waters could reach the surface through a fault (Table 2) (Fig. 10). The fractured zone is assigned a high permeability in Z direction of m 2. To reconstitute the high temperature anomaly observed at UZ-4, we assigned a low permeability of m 2 in the 200 m upper part of the second portion of Material 2 (Fig. 8; which corresponds to Material 8 in the numerical model) at slice No. 26. Fig. 11 Temperature profile of UZ-2, comparison between measured and simulated results. Simulated pressure results are also presented Fig. 12 Temperature profile of UZ-4, comparison between measured and simulated results. Simulated pressure results are also presented The hot fluid is concentrated in the lower bed of the Layer 1. Many combinations of temperature and pressure were used to achieve a good matching between measured and calculated temperatures. The results with the comparison analysis are presented in Figs. 11 and 12. Figure 10 presents the flow pattern at Obama Geothermal Field in different plans. 7 Conclusions Simulating the Obama geothermal aquifer using TOUGH2 (EOS3) leads to a better understanding of the observed processes. The simulated temperature profiles agree with data measured at Obama. The results show that there are mainly two inflows of geothermal water at the bottom of the aquifer and at the eastern boundary of the model laterally. The two inflows may prevent seawater from entering the Obama geothermal aquifer. A conceptual model of the Obama geothermal field has been presented based on current knowledge of the geology, hydrology, and chemistry of the system. From the numerical simulation results, we present the hydrothermal fluid flow model at Obama geothermal field, which is well defined in Figs. 8, 9, and10. The reservoir consists of Obama volcanic rock. The superficial part of the reservoir has a direct connection with the cold sea water interface. Many surface manifestations can be seen at the surface (geyser, hot springs). The groundwater is in most cases of meteoric origin. However, in this area of Obama, it is partially marine. The heat source is considered to be magmatic. The main magma chamber is situated at around 15 km depth beneath Chijiwa bay. Other shallower magma

13 Comput Geosci (2011) 15: chambers were identified beneath WUHTB, Unzen fumarolic field, and Unzen volcano. The temperature of the Obama geothermal reservoir from geochemical studies is around C. The Obama hot fluids are also mixed with cool water coming from the infiltration of surface water and the groundwater flow. The Obama geothermal field can be used much more for various kinds of geothermal energy utilization. Acknowledgements The first author would like to thank Dr. Daniel Hayba of the U. S. Geological Survey and an anonymous referee for their careful reading of the first version of our manuscript and numerous constructive suggestions that helped improve this paper. The authors would also like to thank Dr. Marcelo J. Lippmann (Lawrence Berkeley National Laboratory, USA) for editing the manuscript. Some of the figures were prepared using Generic Mapping Tools [22]. Special thanks go to Dr. K. Fukuoka (West JEC, Fukuoka, Japan) for his guidance during the TOUGH2 model calibration. References 1. Croucher, A.E., O Sullivan, M.J.: Application of the computer code TOUGH2 to the simulation of supercritical conditions in geotermal systems. Geothermics 37, (2008). doi: /j.geothermics Fujumitsu, Y., Ehara, S., Oki, R.: Geothermal fluid flow model in Shimabara Peninsula. J. Geotherm. Res. Soc. Japan 28(4), (2006). (in Japanese with Abstract in English) 3. Kamata, H., Kodama, K.: The volcanic zone as volcanotectonic depression and its formation tectonics-three tectonic events caused by subduction of the Philippine Sea plate under the junction of the Southwest Japan Arc and Ryukyu Arc. Mem. Geol. Soc. Japan 41, (1993) 4. Matsushima, T., Kohno, Y.: Relation between Chijiwa caldera and activity of Unzen volcano. Monthly Earth 28(2), (2006). (in Japanese) 5. Nakada, S., Uto, K., Eichelberger, J.C., Shimizu, H., Sakuma, S.: Investigation of eruption mechanisms through scientific drilling into a volcanic conduit. Proposal to International Continental Scientific Drilling Program (2001) 6. NEDO: New Energy Developing Organization, Geothermal development research document, Unzen Western Region, New Energy Developing Organization, No. 15, 1060 pp (1988) (in Japanese) 7. Ohsawa, S.: A hydro-geochemical study on hydrothermal system and formation processes of hot spings in Unzen graben of the Shimabara Peninsula, Kyushu, Japan. J. Geotherm. Res. Soc. Japan 28(4), (2006). (in Japanese with Abstract in English) 8. Ohta, K.: A study of hot springs on the Shimabara peninsula. The Science Reports of Shimabara Volcano Observatory, the Faculty of Science, Kyushu University 8, 1 33 (1973). (in Japanese) 9. Ohta, K.: Hydrothermal system at Unzen volcano and its geological background. J. Geotherm. Res. Soc. Japan 28(4), (2006). (in Japanese with Abstract in English) 10. Pruess, K.: Development of the general purpose simulator MULKOM. Annual Report Report LBL-15500, Lawrence Berkeley National Laboratory, Berkeley, CA, USA, pp (1983) 11. Pruess, K.: TOUGH2 a general purpose numerical simulator for multiphase fluid and heat transfer. Report LBL , Lawrence Berkeley National Laboratory, Berkeley, CA, USA, 102 pp. (1991) 12. Pruess, K., Oldenburg, C., Moridis, G.: TOUGH2 User s Guide, Version 2.0. Earth Sciemces Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, pp. 198 (1999) 13. Pruess, K.: The TOUGH codes A family of simulation tools for multiphase flow and transport processes in permeable media. Vadose Zone J. 3, (2004) 14. Saibi, H., Ehara S., Fujimitsu Y., Nishijima J.: Geochemistry and geothermometry of hot waters from the Obama geothermal field, between 1984 and In: 2nd International Workshop on Earth Science and Technology, pp Kyushu University, Fukuoka, Japan (2004) 15. Saibi, H., Ehara, S., Fujimitsu, Y., Nishijima, J.: Variations in geothermometery and chemical composition of hot spring fluids in the Obama geothermal field, southwestern Japan. In: Proceedings of the 4th International Workshop on Earth Science and Technology, pp Kyushu University, Fukuoka, Japan (2006b) 16. Saibi, H., Nishijima, J., Aboud, E., Ehara, S.: Euler deconvolution of gravity data in geothermal reconnaissance; the Obama geothermal area, Japan. Butsuri Tansa 59(3), (2006c) 17. Saibi, H., Nishijima, J., Ehara, S.: Reservoir monitoring by repeat microgravity measurement at Obama geothermal field, southwestern Japan: Geothermal and Volcanological Research Report of Kyushu University, N. 14, pp (2005) 18. Saibi, H., Nishijima, J., Ehara, S.: Three-dimensional gravity analysis with the effects of causes of gravity change First phase of the development of gravity post-processor for the computer model HYDROTHERM. Geothermal and Volcanological Research Report of Kyushu University, N. 15, pp (2006a) 19. Saibi, H., Ehara, S.: Temperature and chemical changes in the fluids of the Obama geothermal field (SW Japan) in response to field utilization. Geothermics 39, (2010). doi: /j.geothermics Sugimoto, T.: Geology and petrology at Shimabara Peninsula, Kyushu, SW Japan from recent results. J. Geotherm. Res. Soc. Japan 28(4), (2006) (in Japanese with Abstract in English) 21. Tsukuda, E.: Is the Central Kyushu really rifting north to south? Mem. Geol. Soc. Jpn. 41, (1993) (in Japanese with English abstract) 22. Wessel, P., Smith, W.H.F.: New, improved version of Generic Mapping Tools released. EOS Tran. Amer. Geophys. U. 79(47), 579 (1998)