FLUID INCLUSION MICROTHERMOMETRY AND GAS CHEMISTRY IN GEOTHERMAL SYSTEM, JAPAN AND ITS APPLICATION FOR THE STUDY OF FLUID EVOLUTION - Energy Research Developments: Tidal Energy, Energy Efficiency and Solar Energy - page 180

Figure 13 illustrates the relationship between temperature and CO 2 content of liquid-rich
inclusions in anhydrite from wells M11 and M12 and of steam from wells M6 and M7 drilled
near well M11. The CO 2 contents decrease just along a vapor-loss curve with temperature. As
the inclusion liquids from depths of 850 and 916 m in well M11 have undergone more
extensive CO 2 degassing than the liquid trapped at 1097 m depth in well M12, the reservoir
fluid seems to have flowed upward with vapor-loss during the formation of anhydrite. The
relationships in figure 13 moreover reveal that the present-day steam could have formed as a
result of the recent heating of degassed fluids similar to those from well M11.
The CO 2 and CH 4 contents of the inclusion fluids in anhydrite from well M11 are slightly
higher than those in the steam discharged from the well (table 1; Yoshida and Ishizaki, 1988).
In contrast, the N 2 contents of the inclusion fluids are one order of magnitude higher than that
in the steam. Figure 14 illustrates the correlation between CO 2 /CH 4 and CO 2 /N 2 ratios of the
inclusion fluids in anhydrite from wells M11 and M12. As the CO 2 /N 2 ratios in the inclusion
fluids increase roughly along a vapor-loss curve with CO 2 /CH 4 ratios, vapor-loss due to
boiling provides a reasonable explanation for the variations in the gas ratios.
Figure 14. Plot of log (X CO2 /X N2 ) versus log (X CO2 /X CH4 ) of liquid-rich inclusions in anhydrites from
wells M11 and M12 of the Matsukawa geothermal field (Muramatsu et al., 2006). The schematic vapor-
loss curve was calculated for a single-step separation with adiabatic cooling by assuming the reservoir
liquid contained 2.6 mol % CO 2 , 0.5 mol % N 2 and 0.03 mol % CH 4 at 240 ºC (open star).
3.3.3. Kakkonda Geothermal Field
The Kakkonda I geothermal power plant (50 MWe) has been in operation since 1978,
afterward the Kakkonda Ⅱ geothermal power plant (30 MWe) began to generate in 1996. In
the Kakkonda geothermal field, more than 70 geothermal wells ranging from 500 to 3000 m
in depth have been drilled, and furthermore the deep geothermal exploration well WD-1a was
drilled from 1994 to 1995 to a depth of 3729 m, deepest of all geothermal wells in Japan as a
principal component of the Japanese national project for the “Deep-Seated Geothermal
Resources Survey” by NEDO. Fluid inclusion from these wells has been researched by