Chemical composition of deep hydrothermal fluids in the Ribeira Grande geothermal field (São Miguel, Azores)

详细信息    查看全文

• 作者：M.R. Carvalho ; V.H. Forjaz and C. Almeida
• 关键词：geothermal aquifer ; deep-fluid composition
• 刊名：Journal of Volcanology and Geothermal Research
• 出版年：2006
• 出版时间：1 August 2006
• 年：2006
• 卷：156
• 期：1-2
• 页码：116-134
• 全文大小：304 K

文摘

The Ribeira Grande geothermal field is a water-dominated geothermal system, located within Água de Pau/Fogo Volcano in the central part of the São Miguel Island. This geothermal system is exploited for energy production by wells sustaining two power plants. The wells produce from a formation of pillow lavas divided into different aquifers, with a fairly isothermal zone from 800 to 1300 m in depth, where reservoir temperature reaches 230 to 245 °C. Below the depth of 1300 m there is a slight temperature reversal.

The fluid produced has excess enthalpy and, separated at atmospheric pressure, is characterized by mineralization of sodium-chloride type up to 6–7 g/l, the concentration of dissolved silica varies between 450 and 650 mg/l and the pH ranges between 8 and 8.6. The gas phase is dominantly CO₂, at a concentration of 98 % of NCG.

The composition of the deep geothermal fluid was obtained by computer simulation, using the WATCH program, and was compared with the composition of the bottom-hole samples. The approximations, in this simulation, were considered the single- and multi-step steam separation. The reference temperatures were based on: (i) the measured temperature in wells; (ii) the Na/K geothermometric temperature and (iii) the enthalpy-saturation temperature.

According to both the measured and geothermometric temperatures, the deep fluid of the wells has two phases with a steam fraction up to 0.34, at higher well discharges. The measured enthalpy is always greater than the calculated enthalpy. The calcite equilibrium indicates scaling, since the fluid is flashing, around 2.28 mg/l CaCO₃ at the maximum discharge.

The geothermal wells exploit three different aquifers, the lower of which is liquid and slightly colder than the upper ones. The intermediate is a two-phase aquifer with a steam fraction up to 0.081. The upper aquifer is probably of steam phase. The main differences between the aquifers are the temperature and boiling; both enthalpy and flashing increase as aquifer depth decreases.