Fluid-Mineral Equilibrium at Las Tres Virgenes B. C. S. (Mexico) Geothermal Reservoir

Abstract

The Las Tres Vírgenes geothermal field is a liquid-dominated resource located in the Baja California Peninsula (México) that currently has an installed power capacity of 10 MWe. Exploitation of this field has been limited due to a large, mostly calcite scaling potential of produced fluids which makes necessary the use of commercial inhibitors that suppress calcite and anhydrite deposition, in most of the wells. In this work equilibrium modeling of fluids from the Las Tres Vírgenes wells (from 150 to 300°C) was performed and the saturation indices of minerals were obtained. The results were used to relate the reservoir fluid chemical speciation at equilibrium to suitable mineral stability diagrams to investigate which phases control the chemical composition of fluids. The behavior of the saturation indices of minerals vs temperature for the wells suggested more than one fluid-mineral equilibrium temperature, indicating the occurrence of multiple fluid entries. One of these mineral equilibrations occurs at ~180°C in all the wells, supporting the entry of lower temperature waters to the reservoir. We conclude that observed scaling phenomena in these wells is probably enhanced by mixing of different fluids in the well. Stability diagrams for selected minerals show that the Las Tres Vírgenes fluids have fully equilibrated with the albite-K-feldspar phases. The LV-1 and LV-4 fluids show equilibration with the zoisite-K-feldspar phases at 270°C. The fluids from the wells LV-11 and LV-13 plot in the zoisite stability field at the estimated reservoir temperatures (290- 295°C) but show zoisite-K-feldspar equilibration at 260-275°C. Such temperatures agree with those predicted by the Na/K and Na-K-Ca geothermometers.