When cold fluid flows into fractures within a rock, the rock cools down and the cooling causes thermal contraction of the rock around the fractures. As a result, the fractures are likely to open and to be more permeable. We analyze the effect of remote compressive stresses on fracture opening associated with cold fluid flow by using a 2D FEM code. The results show that the fracture opening will be suppressed by remote compressive stresses, but even in such a condition, the considerable increase in fracture opening and permeability will appear when the temperature of injected fluid is smaller than a critical value Tc. This value is given as a function of remote compressive stresses, fracture pressure, and elastic properties of rock. In order to verify the theoretical prediction, we carried out laboratory experiments using a cylindrical specimen. There exists an artificial fracture passing through the specimen in the axial direction, and the experiments were proceeded as follows ; (i) apply a confining stress to the specimen, (ii) elevate the specimen's temperature to a certain level, (iii) inject cold water into the fracture by a constant pressure, and measure the injection rate, (iv) estimate the fracture permeability from the measured injection rate. From the results of experiments, we confirmed that the fracture permeability increases gradually with decreasing the injection temperature, and the increasing rate changes dramatically when the injection temperature becomes lower than the critical value Tc as theoretically predicted.
We obtained 16 temperature-depth profiles and other hydrological data by field measurement in the Sendai Plain, Northeast Japan to understand hydrogeological regime and subsurface thermal effects. Subsurface temperature in the Sendai Plain is around 14°C in the whole Quaternary system and its thermal gradient is very small, while the Tertiary system has a larger thermal gradient of 3.5°C/100m. Chemical compositions and stable isotopes data (δ D and δ18O) of the groundwater indicate that the groundwater flow system has marked difference between in the Tertiary and the Quaternary systems. In the Quaternary system, where the groundwater flows in a local scale, downward flux caused by groundwater recharge is dominant. Therefore, downward heat transport due to advection from surface may constrain upward terrestrial heat flow. Moreover, subsurface temperature distribution in the Quaternary system is hardly distorted because groundwater flows to the Sendai Bay horizontally. In the Tertiary system, on the other hand, groundwater flow system is regional and upward terrestrial heat flow is dominant factor in the subsurface thermal regime. 3-D groundwater flow and heat transport model of the Sendai Plain shows that the change of thermal gradient near the basement of the Quaternary system can be explained by a hydrological effect.
Accurate understanding of deep geologic structures is very important for characterizing and evaluating geothermal reservoirs. This study aims at three-dimensional interpretation of geothermal reservoirs by a magnetotelluric (MT) survey, inversion of apparent resistivity, and spatial interpolation of one-dimensional (1D) resistivity models. The western side of Mt. Aso crater, central Kyushu, southwestern Japan, was chosen as a study area. The MT measurement was carried out at 26 sites and the data were processed by the remote reference method to reduce local noises. Based on generally low values of skewness of the impedance tensor, local geologic structure at each site was approximated as one-dimensional. Therefore, 1D inversion was applied to the MT data at each site. Resultant 1D resistivity models of all sites were then interpolated by the three-dimensional optimization principle method. The resistivity distribution revealed some continuous conductors of less than 10 Ω·m near hot springs. These conductors may correspond with cap rocks, because the resistivity decreases largely with impermeable clay minerals that are common in cap rocks of geothermal reservoir systems. Thus, two geothermal reservoirs, whose shapes were estimated to be pillars, were detected under the cap rocks at an elevation range from -1000 to -3000 m. By comparing the resistivity model with the temperature distribution computed by a fluid-flow simulation at the steady state, the location and dimension of the estimated reservoirs were validated.
One of the major features of geothermal resources is its wide variety of existing forms. Hence, it is necessary to develop many types of heat mining methods and to pursue their possibilities. The authors have proposed the Downhole Coaxial Heat Exchanger (DCHE) system for exploitation of undeveloped geothermal resources such as Hot Wet Rock, Super Hot Rock, magma origin fluid systems and magma. The major features of the DCHE include the utilization of a highly insulated inner pipe, reverse circulation (i.e., cold water down the annulus and hot water up through the inner pipe) and a completely closed system. Through a heat extraction experiment carried out on the Island of Hawaii in 1991, it was demonstrated that a highly efficient DCHE could be constructed. The authors have carried out two case studies on small-scale power generation with a 2, 000 m class DCHE by numerical simulations. In the first case study, the operational behaviors of the DCHE or the power generation system were investigated assuming the temperature profile and the structure of a well in Hijiori, Japan. In this case, two cases where the binary or the Kalina cycles are combined with the DCHE were investigated. In order to estimate the possible order of the net thermal output of the DCHE or the power output of the power generation system, the second case study was carried out for a higher temperature profile than that in Hijiori. In this case, a temperature profile from Toyoha, Japan was assumed. Through this study, it has been indicated that minimizing pumping power for circulating water in the DCHE is very important for realizing functional power generation. Hence, an appropriate DCHE design is required. A power generation plant which allows a wide range of temperature difference between the hot water and the re-injection water is preferable for combination with the DCHE. Also, It has been shown that 70 kWe class power generation might be possible at Toyoha, Japan with a DCHE 2, 000 m deep.
Subsurface temperature-depth profile preserves the variation of surface temperature, and thus it is considered as an excellent index of the climatic change. To reconstruct the climatic change based on temperature-depth profile, it is necessary to evaluate the effects of the land cover/use changes and of heat advection due to groundwater flow. The purpose of this study is the evaluation of the effects of surface temperature environment and groundwater flow on the subsurface thermal environment in the Kamchatka where human activity negligibly affects subsurface thermal environments. We carried out observations of subsurface temperature-depth profile, the long-term monitoring of soil temperature and analysis of the isotopic ratio of hydrogen and oxygen of water samples. The lithology in the study area consists of the alluvium and sedimentary rocks of the Cretaceous. They have different groundwater flow systems as clarified by the isotopic ratio of oxygen and hydrogen. This difference is also suggested by the temperature-depth profile, which shows the existence of the upward groundwater flow in the alluvium. On the other hand, the annual variation of soil temperature is classified into the large variation period and the stable period. The former is period when the surface is not covered with snow and the latter is covered with snow. The daily change is not observed and temperature is higher than 1.1 degree Celsius in the stable period. It suggests that the increase of subsurface temperature within the shallow zone (>30m) is caused by the rising of the mean air temperature in the non-snow period (large variation period) due to the climatic change, i.e., global warming. Therefore, subsurface temperature is useful to evaluate global environmental change in the high latitude region.
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