Journal of the Geothermal Research Society of Japan
Online ISSN : 1883-5775
Print ISSN : 0388-6735
ISSN-L : 0388-6735
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Article
  • Sho TAKEYAMA, Ryota GOTO, Noriaki WATANABE, Kiyotoshi SAKAGUCHI, Noriy ...
    Type: Article
    2021 Volume 43 Issue 2 Pages 53-64
    Published: April 25, 2021
    Released: October 25, 2021
    JOURNAL FREE ACCESS

    To clarify the characteristics of induced-injection slip under high temperature and the influence on permeability, injection-induced slip experiment using cylindrical specimen with a 45 ° tilted tensile fracture was conducted under the condition 200 - 500 °C. As a result, two types of slips, initial slip and stable slip, were observed under all temperature conditions. The initial slip velocity was constant under brittle conditions and tended to decrease with increasing temperature under ductile conditions. On the other hand, the stable slip velocity decreased continuously with increasing temperature. In the case of granite, the friction coefficient at the start of stable slip tended to increase at temperatures over 400 °C. This suggests that in the ductile condition, the contact area of the fracture surface increases with increasing temperature and the fracture becomes less likely to slip. Additionally, the slip experiments using basalts suggest that under the brittle condition, slips may occur by the same mechanism even if the rock types are different. The permeability after the slip experiment was sufficiently large under the brittle condition but decreased rapidly near the brittle-ductile transition condition and decreased to half at 500 °C. Under ductile conditions, the improvement of permeability due to slip cannot be expected, but the permeability did not decrease to one order of magnitude. Therefore, it is thought that if three-dimensionally distributed fractures with sufficient permeability can be generated in advance, permeability that can perform well as a geothermal reservoir can be maintained. In addition, it was found that the initial slip velocity depends more on the stress field than on the temperature. However, the initial slip velocity under the conditions of 400 °C and normal stress of 55 MPa was a sufficiently small value of 2 μm/s. Also, even if a slip occurrs, it shifts to a stable slip with a lower slip velocity, and it is considered that seismic slip is unlikely to occur under high temperature and high pressure environment.

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  • Keishi NUNOHARA, Hiroki OKANO, Ryoichi YAMADA, Nobuo HIRANO, Noriyoshi ...
    Type: Article
    2021 Volume 43 Issue 2 Pages 65-78
    Published: April 25, 2021
    Released: October 25, 2021
    JOURNAL FREE ACCESS

    The Sanzugawa Caldera had formed through Neogene to Quaternary in south-east part of Akita Prefecture, Northeast Japan. The southern part of the caldera has been investigated for understanding geothermal energy potentiality. We carried out geological survey as a geothermal exploration, and behaviors of thermoluminescence (TL) of quartz were also applied to evaluate thermal effects. According to natural geothermal activities, TL intensity was decreased as a function of time and temperature.

    Three types of hydrothermal alteration were identified in this area, which were neutral pH alteration consisted of montmorillonite, sericite and their mixed layered clay minerals, acid sulfate alteration composed of kaolinite and alunite, and alkaline alteration characterized by laumontite and wairakite. The latter two types of hydrothermal alterations were observed sporadically and correlated to low TL intensity.

    Additionally, TL intensities of samples at high elevation showed relatively high and those of low elevation samples were decreased. Some samples including specific alteration minerals such as laumontite and wairakite showed relatively low TL intensity against general relation between TL intensity and elevation. On the other hand, neutral pH hydrothermal alteration distributed through whole area and showed no clear relation to TL behaviors.

    Those facts strongly suggested that two stages of geothermal activity occurred in this area. First, a neutral pH alteration happened contemporaneous with the Torage-yama formation within the southern part of the Sanzugawa Caldera, and then both of alkaline and acid sulfate hydrothermal alterations were formed by later geothermal activities, showing low TL intensities.

    General trends of TL behaviors which were related to elevation of surface suggest that potential heat source maybe widely distributed in subsurface, and sporadic hydrothermal alteration during later stage hydrothermal alteration associating with relatively low TL intensities was enhanced by upwelling flow through fractures. The latest geothermal activities were related to a discrete magma emplacement, which also indicates potential formation of deep-seated geothermal reservoirs.

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Short Note
  • Mitsuo MATSUMOTO, Kazuki SAWAYAMA, Tsubasa KOGA
    Type: Short Note
    2021 Volume 43 Issue 2 Pages 79-83
    Published: April 25, 2021
    Released: October 25, 2021
    JOURNAL FREE ACCESS

    An analogue experiment using a thermal conductivity meter has been utilized for education and training of well test analysis on the analogy between pressure diffusion in a reservoir and thermal diffusion in a solid sample. The experiment intends to provide students and trainees with an opportunity to validate their understandings after learning the fundamental theory of well testing based on subsurface fluid flow. Several plots of temperature changes generated experimentally exhibit typical curves consistent with pressure changes observed in real well tests successfully. The effects of skin and wellbore storage have been simulated successfully, which can be applied to matching with theoretical type curves.

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