Fluid flow patterns calculated from patterns of subsurface temperature and hydrogeologic modeling—example of the Hohi geothermal area, Kyushu, Japan

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~ _ ~ DI R E;CT •

JOURNAL OF

GEOCHEMICAL EXPLORATION

Journal of Geochemical Exploration 7 8 - 7 9 (2003) 2 7 3 - 2 7 7 www.elsevier.eom/locate/j geoexp

Abstract

Fluid flow patterns calculated from patterns of subsurface temperature and hydrogeologic modeling example of the Hohi geothermal area, Kyushu, Japan Shiro Tamanyu* Geological Survey of Japan, AIST, Site C-7, 1-1-1 HigashL Tsukuba, Ibaraki 305-8567, Japan

Abstract Subsurface temperature and fluid flow vectors have been calculated in a broad sense on the basis of borehole temperature logging data for the Hohi area, Kyushu, Japan. The thermal features suggest two kinds of heat sources for present geothermal systems: one is the Young volcano related geothermal system, and the other the inferred Neo-Granite related geothermal system. The fluid flow vectors are described by numerical simulation based on geometrical parameters, permeability distributions inferred from geological modeling, topographic features, subsurface temperature and so on. The fluid flow in the Quaternary formations is mainly controlled by hydrothermal convections driven by the topographic gradient and subsurface permeability contrast rather than by subsurface temperature gradient. On the contrary, conductive heat transfer is dominant in the pre-Tertiary basement. The lateral floWs from the top of the mountain to both foothill sides are well reconstructed. However, the details of production reservoirs have not yet been reconstructed in this study. © 2003 Elsevier ScienceB.V. All rights reserved. Keywords: Subsurface temperature; Fluid flow; Numerical simulation; Hohi

1. Introduction Regional fluid flow can be estimated from topography, subsurface temperature and permeability distributions, and modeled in detail using a ntunerical simulator of heat and two-phase fluid flow. The Hohi geothermal area, Kyushu, Japan, is chosen as the model field because many drill holes exist and subsurface temperature and permeability distributions are

reasonably well reconstructed. The Hohi area occurs in the western part of a volcano-tectonic depression, named the Kuju-Beppu Graben, which is about 50 km long and up to 40 km wide and is characterized by a low gravity anomaly zone. Two geothermal power plants are located at Otake and Hatchobam in the Hohi area, and they generate electric output of 13 and 110 MW, respectively.

2. Thermal features in the Hohi area * Tel.: +81-298-61-3737; fax: +81-298-61-3717.

E-mail address." [email protected] (S. Tamanyu).

The subsurface thermal features have been explored by many drill holes. In general, subsurface

0375-6742/03/$ - see front matter © 2003 Elsevier Science B.V. All rights reserved. doi: 10.I016/S0375-6742(03)00041-4

274

Abstract

Neo-Granite related GeothermalSystem

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Fig. 1. Geothermalconceptualmodel for the Hohi area (Tamanyu,1991,partlyrevised).

temperature of the shallow part (< 1-km depth) is higher on the horsts than on the graben (buried calderas). The isothermal contours of deep subsurface temperatures are concordant with subsurface relief of the pre-Tertiary basement (MITI, 1987). The conceptual model from northwest to southeast is shown in Fig. 1. The thermal patterns indicate that hydrothermal convection is dominant in the Quatemary formations, and conductive heat transfer is dominant in the pre-Tertiary basement. The Quaternary formations play the role of a permeable aquifer for fluid flow, while the pre-Tertiary basement acts as an impermeable media, except where there is fracture permeability (Tamanyu, 1994). The top surface of pre-Tertiary formation can be estimated by analysis of Bouguer gravity anomaly data with reference to bore hole control points. The deep subsurface temperature pattern in pre-Tertiary basement reflects the underlying geothermal heat sources (Fig. 1). The southeastern high temperature zone is related to young (< 0.3 Ma) andesitic volcanism, where I suggest that heat may be

provided by relatively deep and perpendicular magma chambers. The broad northwestern high temperature zone is related to older (0.3-0.6 Ma) dacitic volcanism, and it may be that the heat sources are provided by high-level consolidated magma plutons. This speculation implies that the magma chambers related with the old volcanism still play an important role as the heat source for present geothermal systems, perhaps equally as well as the younger volcanism-related magma chambers (Tamanyu, 1991).

3. Fluid flow patterns in the Hohi area

A numerical simulation code for coupled heat and two-phase fluid flow was developed for 3D simulation of fluid flow in porous media by Nikko Exploration and Development (Yamaishi et al., 1987). The spatiotemporal changes of fluid pressure, mass flux, flow rate and temperature can be calculated. This simulator was adopted for the convenient calculation of fluid

275

Abstract

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Fig..2. Cross-sections from South to North (D-IY line) through Hatchobaru and Otake geothermal fields, which are near well site HT5-1. (A) Map showing the geologic codes allocated for all meshes with different darkness. The permeability allocated for meshes is shown in Table 1. (B) Isothermal contour map. (C) Distribution map of fluid flow vectors.

276

Abstract

flow vectors along specific cross-sections by means of subsurface temperature and permeability distributions in the Hohi area. The data files of horizontal and vertical plane gridding are made from positions of cross-sections, boundary condition and topographic data files. The horizontal plane is discretized by 250-m-wide cells with additional 4-kin extensions at both ends, and the vertical plane by 100-m-thick cell intervals from surface to - 2000 m asl and larger intervals step by step with depth until - 5000 m. The values of permeability and porosity are allocated to all meshes based on hydro-geologic criteria: permeable formations, less permeable formations (cap rocks) and pre-Tertiary basement (subdivided by water critical temperature, 374 °C) (Fig. 2A). The permeability is fixed as 10 15 m 2 for permeable formations, 10- 17 m 2 for poor permeable formations (so-called cap rock), 10- 17 m 2 for pre-Tertiary formation lower than 374 °C and 10 19 m 2 for preTertiary formation higher than 374 °C, respectively, with reference to the conventional reservoir simulation (Table 1). Subsurface temperature distribution was calculated by the relaxation method, and it is adopted for the fluid flow simulation as fixed temperature data (Fig. 2B). This assumption poses no problem in the case of slow fluid flow that satisfies the heat equilibrium between host rock and fluid. The initial pressure is assumed as hydrostatic pressure. The temperature of surface is fixed as 12 °C. The boundaries of both sides and bottom are impermeable and insulated against mass and heat transfer. However, the 4-km extensions on each side are set to avoid artificial edge effects in the simulation results.

Table 1 List of geologic codes allocated for meshes along cross-sections in the Hohi area

For the fluid flow simulation, the subsurface temperature distribution is first calculated using 38 borehole logging data, and then fluid flow vectors are calculated along 8 cross-sections. Only D - D ~ line is shown in Fig. 2C. This is the cross-section from southwest to northeast, and intersects the cross-section of the conceptual model at the Hatchobaru field. The comparison between the calculated fluid flow pattern and the existing conceptual model indicates that the Calculated flow vectors are well reconstructed generally by this simulation. The distribution map of fluid flow vectors along D - D ' line suggests that heat transfer in the Quaternary formations is mainly driven by the topographic gradient and controlled by permeability contrast, whereas heat transfer in the pre-Tertiary basement is driven by heat conduction. However, the production geothermal reservoir has not been clearly reconstructed.

4. Conclusion Subsurface temperature contour maps are described for the Hohi area based on the calculation of the relaxation method, and indicate that hydrothermal convection is dominant in the Quaternary formations, and conductive heat transfer is dominant in the preTertiary basement. Meanwhile, the vector map of subsurface fluid flow constructed using a flow simulation model indicates that fluid flow in Quaternary formations is mainly controlled by topography and the subsurface permeability distribution rather than subsurface temperature distribution. The lateral flows from the top of the mountain to both foothill sides are well reconstructed. However, the power station production reservoirs have not yet been reconstructed in this study. A more detailed distribution map of permeability, in particular fracture permeability, should be obtained as input data for a more precise fluid flow simulation.

Geologic codes

Hydro-geologic criteria

Permeability (1112)

Porosity

1 2

Permeable formations Pre-Tertiary basement ( < 374 oC) Pre-Tertiary basement (>374 °C) Less permeable formations (cap rock)

1.0 x 10 15 1.0 x 10 i7

0.15 0.03

Acknowledgements

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1.0 x 10 iv

0.15

Nikko Exploration and Development computed the subsurface temperature grid data by the relaxation method. The author expresses his deep appreciation to them.

3 4

Abstract

References Ministry of International Trade and Industry (MITI), 1987. Survey of large-scale deep geothermal development with regard to environmental conservation. Integrated evaluation report (Hohi area). New Energy Development Organization, 116 pp. Tamanyu, S., 1991. Alternative geothermal heat sources besides the youngest volcanism related magma chamber--examples in the Hohi and the Sengan geothermal areas in Japan. Geothermal Resources Council Transactions 15, 47-51.

277

Tamanyu, S., 1994. Structural controlled reservoirs within the horst and graben structure at the Hohi geothermal area, Japan. Geothermal Resources Council Transactions 18, 611 - 615. Yamaishi, T., Kamata, J., Nomura, K., 1987. Numerical simulation of heat and two-phase fluid flow in fractured geothermal reservoir. Extended Abstract for 77th Annual meeting of The Society of Exploration Geophysicists of Japan, pp. 260 265. Written in Japanese with English.