NUMERICAL SIMULATION OF CHANGES IN MICROGRAVITY AND ELECTROKINETIC POTENTIALS ASSOCIATED WITH THE EXPLOITATION OF THE ONIKOBE GEOTHERMAL FIELD , MIYAGI PREFECTURE , JAPAN

This paper presents the results of a feasibility study to appraise the practicality of using geophysical techniques to detect and characterize subsurface changes induced in geothermal reservoir by field operations. A numerical model of the Onikobe geothermal reservoir (Japan) was developed. The modeling study involved (a) developing a stable solution for the natural state of the field prior to the beginning of field operations in 1975, (b) imposing the known fluid production and injection histories based on field records of well flow rates for the period March 1975 – January 1997, and then (c) assuming that well flow rates remain fixed thereafter, forecasting the future response of the reservoir for an additional ten years. Then, the STAR microgravity and electrokinetic potential postprocessors were applied to the reservoir model to calculate the probable changes that will be observable at the surface using these geophysical survey techniques. The strength and spatial distribution of the signals were estimated and the practicality of detecting such signals in the field was appraised. The Onikobe field appears to be a marginal prospect for the utilization of microgravity measurements for future reservoir monitoring based on these calculations, since most of the major microgravity changes have already taken place in the now-mature field. It was estimated, on the other hand, that substantial changes in surface SP started taking place in about 1983, and that these changes are continuing to the present day as a result of the ongoing temporal evolution of the brine productioninjection scheme. INTRODUCTION The New Energy and Industrial Technology Development Organization (“NEDO”; an agency of the Japanese Government) has carried out numerous projects over the years to promote the development of geothermal resources. Most recently, starting in 1997, NEDO has undertaken a new initiative: “Development of Technology for Reservoir Mass and Heat Flow Characterization”(Horikoshi et al., 1998). As a part of this new project, development of “Integrated reservoir modeling and simulation techniques” (Nakanishi et al., 2000) has been undertaken. This involves developing new analysis techniques for the various geophysical data sets and combining them with traditional reservoir engineering practice. As a part of this effort, we have carried out various “feasibility studies” to appraise the practicality of using these geophysical techniques (particularly the monitoring of changes in microgravity and groundsurface self-potential) to detect and characterize subsurface changes induced in geothermal reservoirs by field operations. These feasibility studies involve (1) adopting a particular mathematical “reservoir model” for the system, (2) using that model to perform calculations of changes in underground conditions over a period of many years of operation, and then (3) using mathematical “postprocessors” (Pritchett (1995), and Ishido and Pritchett (1999)) to calculate the probable changes which will be observable at the surface using the geophysical survey techniques. Pritchett et al.(2000) describe the results of one of these studies for a “hypothetical” (but typical) high-enthalpy geothermal reservoir. This paper presents the results of a similar feasibility study for the Onikobe reservoir system; a mature field which has been supplying power to the grid since 1975. In the study, the strength and spatial distribution of the geophysical signals were estimated and the practicality of detecting such signals in the field was appraised. THE ONIKOBE GEOTHERMAL FIELD The Onikobe geothermal field, located in Miyagi prefecture in Japan, has been developed and produced at a capacity of 12.5 MWe by the Electric Power Development Company (EPDC) since 1975. The Onikobe geothermal wellfield is shown in Fig.1. Abe (1988) and Klein et al. (1990) provide a detailed description of the geological setting and the conceptual model of the field, summarized below. The field is located within the 2 million years old Onikobe caldera (about 8 km in diameter). The younger Katayama structural dome (3km × 0.5km) occupies the southeastern part of the caldera. Figure 1. Production and injection well locations in the Onikobe geothermal field. A triangular 200-meter topographic depression (1.5 km × 0.5 km) has formed on top of the Katayama dome, drained by the southward-flowing Ofuka River. This depression is believed to be a downfaulted block resulting from extensional stresses associated with the formation of the dome. The faults are believed to provide the vertical fluid conduits which charge the Onikobe geothermal reservoir. Areas of extensive surface activity and acid alteration are found along the northern and western sides of the triangular depression (the “Katayama thermal area”). The Onikobe wellfield is situated in the Katayama thermal area. The Katayama thermal area is encircled by other natural thermal discharge areas at radii of two to three kilometers. The present investigation focuses on the “study area” shown in Fig.2, which illustrates these features. The wellfield itself is quite small; the “project area” containing the wellheads and the power station occupies only 0.14 km , although the entire area penetrated by the deviated production and injection wells represents around 1 km. Figure 2. Map of the Onikobe study area. The 24-year performance history of the Onikobe reservoir involves two principal phenomenological characteristics. The first is the evolution of drilling practice to maintain field operations. All of the early production wells were drilled vertically and completed in the “shallow” two phase reservoir, because strongly acidic fluids which were unsuitable for use in the plant were encountered at deeper levels during exploration (Abe, 1988). This early steam production was not sustainable, however, and steam production from the shallow reservoir declined with a continuous decline in enthalpy. Then, in 1980, it was discovered that deeper wells, if deviation-drilled away from the center of the anomaly (see Fig. 1), could also produce substantial quantities of hot liquid brines of near-neutral pH. Accordingly, starting in about 1982, production was shifted to these deeper deviated wells. By 1988, the new deep deviationdrilling program had succeeded in returning the field’s generating capacity to 12.5 MWe, and this level of performance is still being sustained, although several make-up wells have been drilled since that time. The second noteworthy characteristic of the reservoir production history involves significant temporal changes in geochemical compositions of the produced fluids. Temporal shifts in geochemistry have been observed corresponding to changes in the production/injection scheme and reflecting the significant spatial variations of the chemical compositions of the geothermal fluids in the reservoir. These changes appear to reflect the effects of both natural and artificial convection and rock/water interactions, downflows of meteoric surface waters into the underlying reservoir system, and mixing of concentrated reinjected brines into the ambient reservoir waters. THE MATHEMATICAL RESERVOIR MODEL