Geochemical modelling of geothermal fluids - application of the computer program SOLMINEQ.88

Abstract

The investigations were carried out based on hydrogeochemical data of the geothermal fluids and well fluid logs of the well KD 22. This well belongs to the Kizildere geothermal field in Western Anatolia. The aim of the investigation was to apply the geochemical model SOLMINEQ.88 on the reinjection as a tool to predict scaling. The production system, the (imaginary) heat exchanger, reinjection well, and reservoir were respected. To start the modelling, some preparations on the input data were necessary. Applying the pressure logs in dynamic state and respecting the steam pressure of water, the partial pressure of CO2 in the reservoir was calculated to be 42 bar. All ingredients such as steam, CO2, trace gases, and alkaline earth metals, which were lost on the path of up-welling were refilled. The composition of the reservoir fluid was recalculated respecting the steam loss. During up-welling and after silencing of the fluid, the main part of the CO2 releases. Supersaturation occurs with respect to calcite and aragonite. Carbonate scaling is precipitated. During the modelled heat exchange down to 50 degrees C, amorphous silica becomes supersaturated due to the lowering of the temperature. In the model for the reinjection well, silica is supersaturated, as well. The model for the mixing in the reservoir showed, that the supersaturation of amorphous silica disappears immediately after mixing. The pre-calculation of mixtures should be preferred, the mixing option of SOLMINEQ.88 does not produce suitable results every time. The modelling of geothermal fluids discovers the thermodynamic character. It gives no information about kinetic or mass balance aspects. Continental geothermal fluids can be modelled easily applying the Debye-Huckel theory due to their moderate ionic strength. Nevertheless, some problems occur due to the programming of SOLMINEQ.88.