AN ASSESSMENT OF THE GEOTHERMAL ... - Orkustofnun

Report 5 27 Akbar
TABLE 12: Different geothermometer temperatures (°C) for the water samples of Bangladesh
Sample no.
1 Na-K-Ca
2 Na-K
3 Na-K
4 Na-K
4 K-Mg
1 74 95 58 116 84
2 94 127 93 147 95
3 88 118 83 139 52
4 82 106 70 126 89
5 88 114 78 134 76
6 81 103 67 124 81
7 89 110 74 131 101
8 62 82 45 104 57
9 65 76 39 98 37
10 67 76 39 98 64
11 97 81 43 102 76
12 123 120 85 140 77
13 88 79 41 100 53
14 124 110 74 131 84
15 134 133 99 153 79
16 95 90 52 111 59
17 100 84 47 105 71
1 Fournier and Truesdell,1973; 2 Fournier,1979; 3 Arnórsson et al., 1983;
4 Giggenbach, 1988;
5.10 Saturation state of calcite
The solubility of calcite is
retrograde in nature. Boiling of
reservoir waters leads to degassing
of the waters, hence raising the pH
of the solution. Disassociation
constants of aqueous carbon dioxide
(H 2 CO 3 ) and bicarbonates (HCO - 3 )
decrease as well. The result is an
increase in the carbonate ion (CO 2- 3 )
concentrations largely responsible
for calcite saturation levels. The
combined ionic activities of Ca 2+
FIGURE 19: Saturation state of calcite
2-
and CO 3 may lead to
oversaturation of the solution with
calcite. The chemical results of the
waters from the Madhyapara hard
rock mine area indicate that the
reservoir waters are either undersaturated
or close to saturation with calcite at quartz equilibrium temperatures (Figure 21). Flashing of
the aquifer waters increases the saturation state of the waters with calcite and further boiling leads to
oversaturation of the waters. Enhanced boiling results in decreased calcite over-saturation, principally
due to the retrograde solubility of calcite. These results indicate the possibility of calcite precipitation
occurring at temperatures in the range of 100-140°C for the analysed samples.