Hideshi Kaieda

Permeability characterization of the Soultz and Ogachi large-scale reservoir using induced microseismicity

Hydraulic fracturing is a common procedure to increase the permeability of a reservoir. It consists in injecting high-pressure fluid into pilot boreholes. These hydraulic tests induce locally seismic emission (microseismicity) from which large-scale permeability estimates can be derived assuming a diffusion-like process of the pore pressure into the surrounding stimulated rocks. Such a procedure is applied on six data sets collected in the vicinity of two geothermal sites at Soultz (France) and Ogachi (Japan). The results show that the method is adequate to estimate large-scale permeability tensors at different depths in the reservoir. Such an approach provides permeability of the medium before fracturing compatible with in situ measurements. Using a line source formulation of the diffusion equation rather than a classical point source approach, improvements are proposed for accounting in situation where the injection is performed on a well section. This technique applied to successive fluid-injection tests indicates an increase in permeability by an order of magnitude. The underestimates observed in some cases are attributed to the difference of scale at which the permeability is estimated (some 1 km 3 corresponding to the seismic active volume of rock compared to a few meters around the well for the pumping or pressure oscillation tests). One advantage of the proposed method is that it provides permeability tensor estimates at the reservoir scale.

Current status of seismic and borehole measurements for HDR/HWR development

Seismic and borehole measurements provide significant information about HDR/HWR reservoirs that is useful for reservoir development, reservoir characterization, and performance evaluation. Both techniques have been widely used during all HDR/HWR development projects. Seismic measurements have advanced from making passive surface measurements during hydraulic fracturing to making passive observations from multiple boreholes during all phases of HDR/HWR development, as well as active seismic measurements to probe regions of the reservoir deemed to be of interest. Seismic data provide information about reservoir extent, locations and orientations of significant fractures, and areas of thermal drawdown. Recent advances include the ability to examine structures within the seismically active zone using statistics-based techniques and methods such as seismic tomography. Seismic method is the only means to obtain direct information about reservoir characteristics away from boreholes. Borehole measurements provide high-resolution information about reservoir characteristics in the vicinity of the borehole. The ability to make borehole measurements has grown during the course of HDR/HWR development as high temperature tools have been developed. Temperature logging, televiewer logs, and electrical property measurements have been made and shown to provide useful information about locations of fractures intersecting wellbores, and regions where water leaves and enters injection and production wellbores, respectively.

Present status of the Ogachi HDR Project, Japan, and future plans

Abstract CRIEPI initiated the Ogachi project in the northern part of Japan in 1986, to determine the feasibility of an HDR geothermal power plant based on a multi-layer reservoir. During the past eleven years technologies have been developed for surveying underground resources, forming multi-layer reservoirs and evaluating these reservoirs in a 1.4 billion yen study. Future plans include the development of a lifespan evaluation method, the improvement of production rates and finally, the establishment of an actual HDR power plant.

Grounded electrical-source airborne transient electromagnetics (GREATEM) survey of Aso Volcano, Japan

Grounded electrical-source airborne transient electromagnetics (GREATEM), a type of semi-airborne electromagnetics, was used to examine Aso Volcano in south-west Japan, to verify its applicability to surveying deep subsurface resistivity structures. Comparison of the GREATEM resistivity values with those of ground-based transient electromagnetics (TEM) data, repeated GREATEM survey results at the same and different flight heights, and lithologic descriptions indicated that GREATEM can successfully identify underground structures as deep as ~800 m in rugged mountainous areas. An active volcanic region (Naka-Dake crater) was mapped as a low-resistivity zone from the surface to a depth of 100 m. This low-resistivity zone extended to the west-north-west, implying future volcanic activity in this area. Therefore, the GREATEM method is useful for surveying deep structures in large, inaccessible areas, such as volcanic provinces, in a quick, cost-effective way.

Passive seismic monitoring of a stimulation of HDR geothermal reservoir at Cooper Basin, Australia

A team of Japanese researchers, with long experience in passive seismic monitoring in the area of development of hot dry rock (HDR) and hot wet rock (HWR) geothermal systems, carried out seismic monitoring at the Australian HDR site being developed by Geodynamics Limited in the Cooper Basin. The seismic network consisting of 4 near-surface stations and 4 downhole stations detected approximately 32,000 triggers during injection of 20,000 m3 of liquid into granitic basement over 3 weeks. The authors located the events on a semi-realtime basis and the locations were fed back to the pumping side. The locations of seismic events showed sub-horizontal extension of the reservoir to 1,800 m away from the injection well at a depth of around 4,500 m. The heterogeneous source migration suggest that the reservoir was close to a critically stressed state.

CO 2 sequestration into geothermal fields

Publisher Summary This chapter evaluates a possibility to sequester CO2 by the storage of a free CO2 phase and by fixation in carbonate minerals in geothermal fields in Japan. In most Japanese geothermal fields, carbonate-rich formations are observed. Underground disposal of CO2 has been preceded in the world, where the temperature of the CO2 reservoir is less than 100°C. In this condition, the injected supercritical CO2 is thought to be stable because of slow rates of chemical reaction with reservoir rocks. The characteristics of Ca extraction from rocks during reactions with CO2-charged water have been evaluated experimentally. A drill core from the Ogachi hot/dry rock field, Akita, Japan is sampled at a depth of 1061m (grained granodiorites). In this experiment, Ca can be released from both anhydrite and silicates such as plagioclase. To calculate the concentrations of Ca released from silicates, the observed Ca concentrations are adjusted on the basis of Ca/SO4 mole ratios in the solutions. These results indicate that Ca can be released easily from silicates and might be removed as CaSO4 and/or carbonate during CO2 sequestration into geothermal fields. The comparison of the experimental results between one with CO2 and the other without CO2 showed that Ca is easily released by the CO2 charged water.