The effective thermal conductivity of one hundred twenty-eight samples obtained from altered rocks in the green tuff regions has been measured at moist and saturated conditions at 20°C by a transient hot wire comparison method in order to clarify the effect of moisture on the conductivity of the rocks and to characterize the thermal conductivity of solid phase in the rocks. Most of the samples contain quartz, feldspar, calcite, dolomite, chlorite, sericite, chlorite/smectite mixed/layer and mica/smectite mixed-layer, and the quantitative ratio of each mineral contained in the sample is relatively high. The effective thermal conductivity has been evaluated by means of a new three phase equation proposed by the authors recently. The results show that the measured values of the conductivity of the rocks have been correlated with values estimated from the new equation within ±20% for all samples. The thermal conductivity of the solid phase in the rocks has been derived from the measured values by using the new equation, also. Further, using an effective thermal conductivity model for a multi-solid-component system modified from the two phase random model proposed by Woodside and Messmer (1961), the thermal conductivity of each mineral in the solid phase has been obtained as the result of multi-variate analysis on the relationship between the thermal conductivity of the solid phase in the rocks and fractions of individual mineral components in them. Calculated values of thermal conductivity of quartz, feldspar, calcite, dolomite and chlorite in the above mineral group based on the multi system model is close enough to their literature values obtained by Horai (1971). This shows that the effective thermal conductivity equation for the multi-solid-component system proposed by the authors also is applicable to evaluate the thermal conductivity of each mineral in the solid phase of the rock.

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Reinjected water through well contains dissolved air which may isolate to make air bubbles in the thermal reservoir under a certain pressure. Air bubbles in the reservoir obstruct the flow of geothermal fluid to decrease the ability of the reinjection well. In this paper, some of results of the model experiments on air bubbles generating in the porous layer and decreasing the permeability of the layer are shown. Results of the experiment show: (1) appearance of bubbles is more conspicuous when the flow is slow than fast: (2) the bubbles have a tendency to appear at inside rather than at inlet of the porous layer: (3) ten percent increase of air bubbles content causes about twenty percent decrease of relative permeability for water. These results of the model experiment can be applied to the actual reinjection wells when the solubility of air in the geothermal reservoir is less than that at the head of the injection well.

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In our previous paper, we analyzed the stress state in the solidified region of the heat extraction system from molten magma under the condition of plane strain. In the present paper, we treated the same problem by using the three-dimensional axi-symmetric model in order to take properly account of the effect of body force due to gravity which was treated approximately in our previous paper. The stress field is analyzed by using creep theory for three types of rocks, i.e., granite, olivine and anorthosite, to shed light on the variation of stress field with respect to time after the onset of heat extraction. The usual incremental flow theory are employed, where the second invariant of the stress deviator is used as the creep potential. It was revealed that the normal stresses acting in the circumferential and axial directions on the wellbore wall shifted gradually with time to become compressive. Thus, the formation of the fracture network which is expected to be created by thermal shrinkage of rock due to heat extraction and to work as the flow path of working fluid to extract heat from solidified magma, is mostly determined by the stress field in the early stage of the heat extraction. Based on this fact, we propose a concept of a stress map expressed in the T_s vs. Ea diagram (E: Young's modulus, α : thermal expansion coefficient, T_s: solidus temperature). By using the stress map, a rough estimation on the formation of the fracture network is feasible for any kind of rocks. Among the three rocks mentioned above, olivine is the most potential candidate for extracting heat directly from magma.

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