Environmental Engineering Reference
In-Depth Information
Fluid inclusions in transparent minerals are classified into two-phase, vapor-rich and
liquid-rich inclusions, and polyphase inclusions comprising liquid, vapor and solids. As
mentioned in section 2.1, the minimum Th of the inclusions in hydrothermal quartz, anhydrite
and calcite, and igneous quartz from production wells is in good agreement with the measured
borehole temperature after a long standing time in the shallow (1400 m depth) and deep
reservoirs (1400 m depth). Three dimensional distribution of the minimum Th of the liquid-
rich inclusions from many production and reinjection wells suggests that high temperature
reservoir fluid has been ascending from the northwestern depth during the present magmatic
activity related to Kakkonda granite. The upflow zone is presented not only by the three
dimensional distribution of Th value at the surface, but also by three dimensional distribution
of polyphase inclusion. Polyphase inclusion observed in quartz and anhydrite is widespread in
the deep reservoir, whereas is restricted at the northwestern area in the shallow reservoir. The
upper limit of three dimensional distribution of anhydrite corresponds with the up flow zone,
being indicative the upflow zone of the field.
Based on the results and the geological, geochemical and geophysical data in the field,
the exploration well WD-1a was drilled to the direction of the deep reservoir in the upflow
zone at the contact of the Kakkonda granite at a depth near 3000 m. According to Komatsu et
al.(1998), the minimum Th of liquid-rich inclusions in both igneous and hydrothermal quartz
samples from depths of 3700 and 3728 m of the well has a maximum value ranging from 483
to 507 ºC of all wells in the field. The similar temperature was obtained using melting tablets
after standing times of 129 and 159 hours at 3700 m (500 to 510 ºC), and moreover estimated
using Horner plots at 3500 m (501 ºC; Ikeuchi et al.,1998).
Gas Evolution in the Reservoir Fluid
Gas chemistry of fluid inclusion was performed on hydrothermal quartz and anhydrite in
order to discuss gas evolution on the reservoir fluid at the Kakkonda geothermal field (Sawaki
et al., 1999; Muramatsu et al., 2000). Under the microscope, liquid-rich inclusions are mostly
observed on these minerals from the shallow reservoir. But both liquid-rich and vapor-rich
inclusions are observed on anhydrite from the deep reservoir, suggesting that fluid inclusions
were widely trapped under boiling condition. As shown in figure 16, the CO
2
contents in the
liquid-rich inclusions of these minerals decrease with decreasing temperatures. Figure 16 also
reveals that the reservoir liquid was made by differentiation of the initial reservoir fluid due to
vapor-loss at deeper depths, and was made by dilution of the residual liquid with the meteoric
water at shallower depths. The dilution degree increases with decreasing the depth. On the
other hand, the present reservoir fluids in the shallow reservoir are plotted on the higher
temperature side of the vapor-loss curve calculated by a single-step vapor separation,
suggesting that they may be formed through multi-step vapor separations.
It is significant problem when the CO
2
content in the reservoir fluid decreased. Figure 17
shows variation of temperature and CO
2
content of the reservoir fluid from three production
wells drilled in the shallow reservoir with time during about two years after the end of
drilling. Although the steam and hot water discharged from these wells were collected after
stopping of affection of a drilling mud to fluid composition, the CO
2
content of 0.232mol% in
the reservoir fluid at the end of drilling of well-62 is nearly equal to that in the inclusion fluid
(about 0.1 to 0.7 mol%; table 3). As shown in figure 17, after the CO
2
content in the reservoir
fluid from well-62 abruptly decreases during discharge testing of the well, it is constant of
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