The Gaia snow-melting system, which utilizes the geothermal heat and solar energy, was constructed in Ninohe, City, Iwate Prefecture, in 1995. The authors have studied the heat transfer characteristics in the pavement and roadbed surrounding the heating tubes. This is because in the Gaia snow-melting system, the heating tubes have been used to melt the snow on roads. In the present study, a numerical simulation has been conducted to investigate the heat transfer characteristics in the pavement and roadbed. Also, the effect of the insulation block on both the heat flux supplied to road-surface and that lost downward has been discussed. It was found from our computed results that the road-surface heat flux with an insulation block is greater than that without an insulation block when examined several hours after the start of the snow-melting operation. We also found that the insulation block diminishes the heat loss in a downward direction when compared with cases without an insulation block.
The Sumikawa geothermal field is located in the Hachimantai volcanic region of northeastern Japan. The commercial operation of the power plant started in March 1995 with an installed capacity of 50 MWe. There, separated hot water and steam condensate, both are reinjected into the geothermal reservoir. In this paper, the influence of the reservoir on physicochemical characteristics due to reinjection fluids was studied. Changes in the chemical and hydrogen isotopic compositions with time were observed on production fluids during the exploitation. Some wells showed seasonal cyclic changes in Cl concentration and δD, which were inversely correlate It was inferred that such changes were due to return of steam condensate reinjected to production area. This inference was verified from tracer tests. In order to simulate the physicochemical changes in the reservoir caused by reinjection of steam condensate and separated water, a lumped parameter model was developed based on the conservations of mass and energy. As regarding three wells, SB-1, S-4 and SC-1, the model calculation was able to adequately simulate the seasonal cyclic changes in Cl concentration and δD in a production fluid, and also to determine returning ratios of steam condensate and two kinds of separated water reinjected, fluid mass and porosity in a production tank, that means a definite volume around the production well where the reinjected fluids are well mixed up with the original geothermal fluid coming up. The returning ratio of steam condensate is relatively high in the wells in which the seasonal changes were clearly observed. We also found that the returning ratios were different in individual reinjection section. Decreasing rate of the reservoir temperature around a well is dependent on volume of the production tank and the degree of the returning ratios in individual well. Decreasing rate is relatively rapid in the well that has small volume and high return ratio of steam condensate.
It is prerequisite to clarify size effect on fracture permeability to estimate permeability of actual natural fractures or artificially created hydraulic fractures. However, aperture distributions, which are necessary for the estimation, are difficult to be measured for a large fracture. In this study, by creating a synthetic square fracture with various sizes from 41 mm to 656 mm in a side length, size effect on the fracture permeability is investigated by solving Reynolds equation to obtain a hydraulic aperture of the fracture. Synthetic fracture surfaces are createdd with a spectral synthesis method based on fractional Brownian motion (fBm) by giving a gradual phase difference between the Fourier components of the two fracture surfaces. When a fracture is closed to have a certain mean aperture, the hydraulic aperture decreases with the fracture size. This is due to the increase in the percentage of contact point with the fracture size because the standard deviation of the aperture increases with the fracture size. When a fracture is closed to have a certain percentage of contact point, the hydraulic aperture increases with the fracture size. This is, due to the increase in the mean aperture with the fracture size because the standard deviation of the aperture increases with the fracture size. Thus, size effect on fracture permeability is governed by size effect on the standard deviation of the aperture.
An injection test on Sumikawa well SD-1 was carried out to evaluate the effect of water temperature on injection-capacity. It was concluded that injecting low temperature water increases the permeability-thickness product and the injection flow rate owing to increase in fracture aperture and decrease in reservoir pressure around the injection well. To evaluate quantitatively reservoir pressure changes and the influence of injection temperature on the injection-capacity during injection, we have conducted numerical studies using a two-dimensional cross-sectional model. Considering the relationship between porosity, permeability, and pressure/temperature, a modified Kozeny-Carman equation was applied -to a double-porosity (MINC) model, where the fracture-porosity and permeability should increase as a result of cooling in the reservoir. Consequently, it is suggested that the injection-capacity tends to increase during injection due to an increase of permeability caused by an expansion in the fracture aperture, if the temperature of the injected water is significantly lower than that of the reservoir. Since the injected low temperature water tends to floww down vertically through fractures rather than horizontally because of gravity effect, the cooling of the reservoir also propagates in the vertical direction rather than in the horizontal direction. Accordingly, the vertical permeability increases with time leading to a well injection-capacity increment during injection.