| Abstract |
The Rotokawa deep fluid is characterized by near neutral-pH (6-8), low chloride (<800 mg/kg) and high gas content (450-500 mmol/100 mol in total discharge). In the absence of true chloride springs in the area, the chemistry of the wells were the only reliable source of information on the composition of deep reservoir fluids. The Rotokawa model suggests that the upflow of the system is located near RK4, which chemically and isotopically showed the closest affinity to the deep parent water. In relation to its chemistry, the shallow well RK2 tapped a gas-rich, high-enthalpy steam zone in the shallower part of the reservoir. In contrast, the other shallow well RK3, probably tapped a perched reservoir of high chloride but low gas residual fluids. The discharge compositions of deep well RK5 and RK9 fluids appear to be the result of dilution of the parent water by low chloride but high enthalpy (1400-1500 kJ/kg) water, whose source is uncertain. The most dilute discharge composition of RK6 represents the chemistry of the outflow sector of the field. Fluid-mineral equilibria suggest that the deep fluid of Rotokawa is in equilibrium with K-mica, as a result of the high CO2 content of the field, as well as, with quartz, albite and adularia. The deep fluids of RK5 and RK9 were found to be undersaturated with calcite and anhydrite at high temperatures and even at boiling conditions due to the limiting effect of very low calcium in the waters. Silica scaling occurs at relatively high temperatures (200░C), which requires separation at pressures not less than 15.5 bar absolute to prevent silica deposition problems. |