We analyzed tracer tests in the Ogiri geothermal field, which was conducted in 2008 and 2011, respectively. The test results were analyzed with the multi-flowpath model, then interpreted together with hydrological features of the field determined from geological settings and geophysical surveys. The results indicate the presence of two flow path for reinjected water flowing to the production zone in the Ginyu fault. A part of the reinjected water flows into the main production zone of the Ginyu fault directly through this fault from the west. The water also flows the Ogiri deep faults zone located to the south of the Ginyu fault, then flows into the Ginyu fault from the east which takes longer time to reach the production zone.

The author reports a numerical modeling study considering the recovery process of a wellbore water level due to temperature recovery after water injection into a geothermal well. This study intends to estimate the transmissivity of a geothermal reservoir based on this recovery process. We assume that the real recovery process of a wellbore water level after water injection consists of two components: the component of pressure recovery in the geothermal reservoir and that of temperature recovery at each depth in the well. The former component can be estimated by modeling the latter component and eliminating it from the recovery process of a wellbore water level.

The numerical model takes into account heat transfer due to advection in a well and conduction in rocks. Heat exchange between the well and rocks is estimated based on an empirical equation. The numerical techniques and code developed originally for this study are validated by referring to the analytic solutions of simplified problems and heat balance. The numerical solution of the recovering water level is characterized by the slope of the linear trend line defined in the Horner plot. Numerical experiments are demonstrated for revealing the sensitivities of this slope to both the dimensional and dimensionless parameters. An empirical equation is derived based on these numerical experiments.

In ground-source heat pump (GSHP) systems, the application of horizontal ground heat exchangers (HGHEs) greatly reduces the initial costs for the system construction in comparison with vertical GHEs since the HGHEs can be installed using commonly-used excavation machines. Though HGHEs have been widely used in the United States and Canada, where abundant land space is available for HGHEs, the distribution of the system is still slow in Japan due to the limited availability of land space.

In this paper, data analysis and numerical modeling were carried out on a GSHP heating and cooling system using HGHEs installed in the basement of a railway tunnel in Tokyo, Japan. Through the interpretation of operation data for 2 years, the decline of system performance by the change of ground temperature was not observed though the amount of heat release and extraction were not in a good balance.

A numerical simulation model of the HGHE was then developed based on the ground soil properties and HGHE designs. The model was validated using the recorded data during the heating and cooling periods. Sensitivity studies were then carried out using the validated model for the heating and cooling operations of 10 years. The predicted inlet temperature of heat pumps showed that the GSHP system could maintain the initial capacity for 10 years in case the annual heating and cooling periods were set at 4 months and 2-4 month, respectively. Also, the effect of burial depth of HGHE on system performance was examined using the numerical model. The simulations showed that deeper installation is more preferable when the heat exchange rate per HGHE length is large, while the difference is negligible when the heat exchange is relatively small.