Geothermal-well measurements

Ariki, K. Akibayashi, S. 2001. Effects in the injection temperature on injection-capacity of geothermal wells - Numerical simulation on the measured downhole pressure. In 2001 Annual Meeting Geoth. Res. Soc. Japan Abstr. with Programs, Nagoya, 3-5 Dec. 2001 (in Japanese) p. A06. 

The Okuaizu geothermal system is characterized by high temperatures (maximum 340°C), high salinity (about 2 wt% total dissolved solids (TDS)) and large amounts of non-condensable gases (1 wt% CO2 and 200 ppm H2S). The pH of the hydrothermal solution measured at 25°C is 6.44 (Table 2.6). However, the pH of the original fluid in the reservoir is computed to be 4.05. This pH as well as alkali and alkali earth element concentrations are plotted near the equilibrium curve of albite, K-mica, anhydrite and calcite (Fig. 2.19) (Seki, 1991). 

Fig. 2.39. Na /K+ atomic ratios of well discharges plotted at measured downhole temperatures. Curve A is the least squares fit of the data points above 80°C. Curve B is another emperical curve (from Truesdell, 1976). Curves C and D show the approximate locations of the low albite-microcline and high albite-sanidine lines derived from thermodynamic data (from Fournier, 1981). Small solid subaerial geothermal water Solid square Okinawa Jade Open square South Mariana Through Solid circle East Pacific Rise 11°N Open circle Mid Atlantic Ridge, TAG.

Temperatures measured in wells and mines reveal a general increase with depth, or a geothermal gradient (Fig. 4.18). The value of the geothermal gradient varies from one location to another, an average value being 3 °C/ 100 m. 

In the geothermal areas the more positive 5 C(CH4) values are generally observed in the highest temperature wells but the temperatures calculated on the assumption of isotopic equilibrium between CH4 and CO2 are much higher than the measured temperatures. 

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