The air-conditioning systems with groundwater heat pumps using the Aquifer Thermal Energy Storage (ATES) are known to be energy efficient and are well developed in European countries. On the other hand, the number of systems is still few in Japan due to the lack of knowledge of optimum design for the system.

In this research, long-term heating and cooling tests were carried out in an ATES system installed in Japan Groundwater Development Co., Ltd., located in Yamagata City, northern part of Japan. During the tests, field data, i.e., the temperatures and flow rates of the pumped and injected water, performance of heat pump, etc., were recorded and these data were analyzed to demonstrate the high energy efficiency of the system.

Then, based on the geological and groundwater information at the test site, a 3D numerical model was developed using a groundwater and heat transport simulator. Using the numerical model, the long-term performance of the ATES system was predicted based on the climate conditions of Yamagata City. The simulation predicted the formation of the cold water zone in the aquifer, due to the imbalance of the extracted and disposed heat. On the other hand, performance of ATES system could be improved by introducing appropriate placement of wells.

This paper deals with the application of the integrated analysis of gravity and geo-magnetic data on the lava dominated deposited area in the Kuju geothermal prospective region.

The gravity/geo-magnetic relationship in and outside of the depression can be characterized and separated in the classified cross-plots of gravity and pseudo-gravity (BG-PsG) data sets by the PsG/G Ratios obtained through the moving window correlation.

The linear line of the BG-PsG relationship which represents the depositions of 100% lava filling in the depression (Maximum Volcanic Line, MVL) can be drawn in the cross-plots of the BG-PsG data sets. The deviation of the measured PsG ( ⊿PsG) from the MVL relates to the content of lava deposits from the adjacent volcanos in the depression. The low ⊿PsG corresponds to the area of the high lava content, while the high ⊿PsG is located in the area of the low lava content and subsequently the high content of such tuff depositions as rocks of less magnetization. 

The results show two low ⊿PsG trends, and they correlate satisfactorily well to the high geo-temperature areas defined by the prevailing reports and drilled wells; one being from the Otake/Hatchobaru geothermal area extending toward the NW to the area of Sugawara geothermal plant (GP) and the another being from the Mt. Kuenohira, via Takigami(GP) to Noya region. The high ⊿PsG trend lies between them, corresponding to the low geo-temperature area of the Shishimuta depression.

Considering the depositions of lava tend to fill in such low lands as lagoon, lakes, rivers etc., which is paleogeographically the case in the studied area, the hot water passes acting as geothermal reservoir could possibly be formed by the cool joints when the lava flew in the water of such low lands. This type of geothermal reservoir tends to extend horizontally as referred to the prevailing reports and results of drilled wells, and may form additional geothermal prospects in addition to that of conventional reservoir of vertical fractures.

The conventional reservoir simulation model has been extended. This extension is based on an assumption that the highly heterogeneous distribution of pores in reservoirs generates pores containing immobile fluids. The pores containing immobile fluids only allow the propagation of pressure change. The specific enthalpy and tracer concentrations in these pores are maintained at the initial values.

Attaching the pores containing immobile fluids on the outside of the porous medium defined in the conventional model, we assume that the total pore volume increases by a factor of α. The changes in tracer concentrations are controlled by the porosity of the original porous medium ϕ, whereas that in pressure is controlled by αϕ. Numerical experiments validate these dependencies of pressure and tracer concentrations.

An effective procedure for matching the extended simulation model to field observations is discussed in terms of the abovementioned dependencies. A further extension is described by taking into account the thermal decay of multiple tracers.