Jökull


Jökull - 01.12.1987, Side 39

Jökull - 01.12.1987, Side 39
(2) The CO, concentration in the parent water from which the steam is derived is that given by the C02-temperature function for equilibrated reser- voirwaters by Arnórsson and Gunnlaugsson (1985). (3) The N2 concentrations in the parent geothermal wa- ter as well as in the cold shallow water are those of non-thermal water in equilibrium with the atmo- sphere. (4) The C02 and N2 are unreactive in the upflow. (5) Neither C02 nor N2 partition significantly into the condensate/steam heated water. These simplifying assumptions are discussed briefy be- low. It is difficult to provide evidence regarding the validity of the first assumption of the boiling process in the upflow. However, as discussed later, the low N2 concen- trations in the fumarole steam at Krísuvík are taken to be indicative of extensive boiling in the upflow, exten- sive meaning that the water has boiled over a large temperature (pressure) interval (>50°C) without being separated from the steam phase. Drillhole data were used to derive the temperature function for C02 concentrations in geothermal reservoir waters. They show some scatter which may be due to several reasons as discussed by Arnórsson and Gunn- laugsson (1985) including: a) departure from equilib- rium; b) differences in the composition of minerals par- ticipating in the equilibrium; c) imperfections in crystal structures; d) errors in selected aquifer temperatures and e) presence of equilibrium steam in the reservoir. The scatter amounts on average to 0.27 logm CO, units. For 280°C reservoir water such departure would give at the most 9% condensation, if there was actually none. The third assumption is based on the fact that geother- rnal waters are dominantly meteoric in origin and it is valid as long as the cold parent water has approached fairly closely equilibrium with the atmosphere and mag- matic contribution is not significant, or contribution from other sources, and as long as degassing has not occurred prior to boiling in the upflow. This assumption finds support in drillhole data (Arnórsson, 1986). N2 is the dominant nitrogen bearing species in Icelan- dic geothermal systems, including the Krísuvík field. Any reduction that might occur into NH3 or oxidation into N03, will, therefore, not significantly affect the N2 concentrations, suggesting that N2 is effectively unreac- tive. Geothermal reservoir waters are generally very close to being calcite saturated (Arnórsson, 1978; Arnórsson etal., 1983b; Ellis and Mahon, 1977). Upon boiling they become calcite supersaturated (Arnórsson, 1978) caus- ing some removal of carbonate from the fluid, thus lowering its C02 content. Total carbonate concentra- tions in geothermal waters vastly exceed those of calci- um, except for saline waters below some 150°C. There- fore, the amount of carbonate removed from solution by calcite precipitation is only a small fraction of the total carbonate due to limited availability of calcium. Con- densation of steam, whether by conductive heat loss or mixing with cold water, produces an acid and calcite undersaturated aqueous phase. Condensation does not, therefore, lead to removal of C02 from the steam phase through precipitation. Both C02 and N2 are sparingly soluble in water. For a system at 100°C and with 5% steam by weight 99.7% of the mass of C02 occupies the steam phase and over 99.9% of the N2. At 250°C the same figures are 88% for C02 and 98% for N2. It is, thus, evident that only very extensive condensation at high temperatures will cause significant partitioning of C02 into the condensate. Un- der all conditions practically all the N2 occupies the steam phase. The following function describes the relation between N2 concentrations in steam at atmospheric pressure and the temperature of the parent water assuming adiabatic boiling: logN,c = 22.965 - 0.117235 • T + 2.05872 • 10'4 • T2 - 1.22958 • 10*7 • T:i (1) where N2 is in mmoles per kg of steam and T in °K. The subscript c denotes that the N2 concentration is a calcu- lated value to distinguish it from measured concentra- tion in the steam (N2 m). When deriving function (1) it was assumed that the N2 content of the parent water was 0.71 mmoles/kg which corresponds with saturation at 5°C at PNj = 0.78 atm. For a parent water saturated with N2 at 25°C (0.45 mmoles/kg) equation (1) becomes: logN2 c = 22.700 - 0.116818 • T + 2.05013 ■ 10'4 • T2 - 1.22377 • 10“7 • T3 (la) As C02 concentrations in the reservoir water are de- termined by its temperature and the N2 concentrations are taken to be constant, it follows that the C02/N2 ratio in that water is a function of temperature only. The following function describes the temperature depend- ence: t = 148.5 + 64.35 • Qcn + 5.239 • Qcn2 - 1.832 • Qcn3 (2) where Qcn is log(CQ2/N2) in moles and t is in °C. The 37
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